This is the busiest day of the 2010 primary election season, with Democratic and Republicans nomination fights (along with some third party battles and lots of referendums) on the ballots in a dozen states.

Here’s where and what type of voting is taking place:

Arkansas – Primary Run-Off Election 
California – Primary Election
Georgia – CD-9 Special Election Republican Run-Off
Iowa – Primary Election
Maine – Primary Election
Montana – Primary Election
Nevada – Primary Election
New Jersey – Primary Election
North Dakota – Primary Election
South Carolina – Primary Election
South Dakota – Primary Election
Virginia – Primary Election

And here races and trends to watch:

1. Arkansas Democratic U.S. Senate Run-off:

If challenger Bill Halter beats incumbent Blanche Lincoln, the shockwaves will be felt from Little Rock to Washington. The Democratic political establishment has backed Lincoln, with Bill Clinton and Barack Obama uniting to back the conservative incumbent. But Halter has become the vehicle for labor unions and Internet activists to express their displeasure with Democrats who are more prone to compromise than change. Lincoln is a good target in this regard, even if Halter is not quite so progressive as some of his backers imagine. (The Arkansas lieutenant governor is running a populist campaign that wisely seeks to tap into generalized anger at Washington.

If Halter does win, the most important message will be this: Despite all the media frenzy regarding the influence of the Tea Party movement on the Republican primaries, the real action is on the Democratic side. Democratic primaries have generally been attracting higher turnout and they have seen more political turbulence than Republican contests. If Lincoln becomes the second Democratic senator (after Pennsylvania’s Arlen Specter) to lose a primary, the pundits might just wake up to the fact that the most energetic and effective challenge to the status quo is playing out within the Democratic Party.

2.California District 36 (Harman-Winograd):

Progressive challenger Marcy Winograd has scared the wits out of conservative incumbent Jane Harman (every Republican’s favorite Democrat. This race is a real test of anti-war sentiment. Harman was an apologist for the Bush-Cheney administration and has encouraged the worst instincts of the Obama-Biden administration with regard to Iraq and Afghanistan. Winograd is backed by Progressive Democrats of America, Democracy for America and key anti-war activists. She’s also won a good deal of backing from unions that object to Harman’s pro-Wall Street. But Winograd’s determination to hold Israel to account in the aftermath of the raid on the Gaza aid flotilla has given the incumbent a tool to attack the challenger, and many otherwise liberal House leaders (including Henry Waxman and even Congressional Progressive Caucus cio-chair Lynn Woolsey) have backed Harman.

Harman remains the frontrunner, but a Winograd win would signal that Democrats really are frustrated with their party’s caution regarding foreign policy.

3. Is the Tea Party Over?

Tea Party activists and their allies have lost a lot of primaries so far, and they will lose more on Tuesday. A key test will come in California, where right-wing state Senator Chuck DeVore is the favorite of conservative activists. DeVore has compared himself to Jack Bauer of “24” (He can, his ads declare, “throw a mean hand grenade") and run a generally creative and activist campaign. If he were to win, it would be a huge victory for the Tea Party movement and the GOP’s hard right. Glenn Beck tells DeVore "you intrigue me." John Gizzi of Human Events writes: “For conservatives, there is one choice to put up against Boxer: DeVore, U.S. Army Reserve lieutenant colonel, former Reagan Administration official, and the leading spear-carrier on the right in Sacramento…”

But DeVore is likely to lose to millionaire businesswoman Carly Fiorina, who has secured lots of DC –insider backing and spent lots of her own money to gain the lead. If Fiorina wins in California, the state that gave us Ronald Reagan and Proposition 13, is there anyplace where the Tea Party can win? (Yes, yes, I know folks may mention Tennessee Republican Senate nominee Rand Paul. But Paul had a lot more going for him than the Tea Party, including the millions raised by libertarian-leaning followers of his father, Congressman Ron Paul.)

There are lots of other Tea Party tests going on Tuesday, in places like New Jersey, where activist Richard ("This Campaign is in Keeping with the Tea Party Movement") Luzzi is challenging relatively moderate Republican incumbent Rodney Freylinghusen for a GOP U.S. House nomination. Frankly, the movement has to win some of these races if it is going to measure up to its news coverage.

A lot of attention will be focused on the reelection fight of South Carolina Congressman Bob Inglis,, a relative moderate who may not get 50 percent of the vote in his Republican primary race today. If Inglis is forced into a runoff, that could become one of the highest-profile Tea Party tests of the season. The South Carolina run-off, if required would take place June 22.

4. Is Money the Only Thing That Really Matters?

If Halter wins in Arkansas, it will be because he and his backers were able to match the spending on Lincoln’s behalf, thanks to strong backing from unions and groups such as MoveOn. Similarly, if Fiorina wins the California Republican Senate nod, despite the fact that almost everyone agrees she has proven to be an inconsistent contender, it will be because she outspent able opponents, conservative DeVore and centrist Tom Campbell.

And if billionaire Meg Whitman, a ridiculous inept candidate, wins the California Republican gubernatorial nod, then there cannot be much question that money changes everything.

In light of the Citizens United v. FCC ruling by the Supreme Court, which said corporations can spend as freely as they choose, that might be the most unsettling message of the day.

5. Is the Great Hope of the Democratic Party a 72-Year-Old Guy They Used to Call "Moonbeam"?

The biggest race of the 2010 election cycle may well be the contest for governor of California, the nation’s most populated and troubled state. The first thing the new governor will do is manage a redistricting process that could determine which party wins or loses dozens of U.S. House seats. So the California race is a big deal for the state and the nation. The all-but-certain Democratic nominee is Jerry Brown, the former governor and three-time presidential candidate that pundits once dismissed as “Moonbeam.” How well will Brown, who so far has run an oddly casual campaign, do against his little-known foes in the primary. If he runs up a big vote and a high percentage, despite the fact that most of the primary action is on the Republican side of the ballot, he could well be on his way back to the governorship – and the best-laid plans of Republican strategists could be in tatters. Also, keep an eye on the race for the Democratic nod for lieutenant governor, where San Francisco Mayor Gavin Newsom is the supposed frontrunner. Newsom quit the governor’s race to make way for Brown but he remains of fascinating political figure – supremely ambitious yet also courageous on issues such as same-sex marriage.

6. Popular Democracy.

As usual, California has what look to be the most significant referendum votes. There’s a bold proposal for campaign finance reform, Proposition 15, and a foolish proposal to narrow choices in November elections by effectively locking out third parties and independents, Proposition 14. Here’s what the wise folks at the San Francisco Bay Guardian say about the two votes:

PROPOSITION 14 OPEN PRIMARIES NO At the height of a royal mess last year when the state budget was long overdue and the two-thirds majority needed to pass it was still out of reach by one vote, Republican Sen. Abel Maldonado struck a deal with Democrats. He said he’d support the budget — if the majority party would meet a few of his demands. One thing he insisted on was Prop. 14 — a ballot measure that would effectively remove political parties from the primary elections process, allowing all voters to cast ballots for any candidate regardless of party affiliation. Under Maldonado’s plan, all candidates would run on a single primary ballot, and the top two vote-getters would face off in the general election. Heavily funded by the California Chamber of Commerce and marketed by the same spin doctors and corporate lawyers who are rolling in Yes on 16 campaign money, Prop. 14’s backers say it will result in more centrist elected officials. There are plenty of pitfalls here, the most worrisome being that it would drive up the cost of elections and give more moneyed (and corporate-allied) candidates a sharper competitive edge while elbowing out progressives. It would allow Republicans to play a role in what would normally be Democratic primaries (and vice versa.) The measure would also make it nearly impossible for smaller parties — the Green Party, for example — to offer candidates in the November elections. Bad idea, bad process, Vote no. PROPOSITION 15 FAIR ELECTIONS ACT YES California desperately needs electoral reform. Corporate campaign spending and lobbyists have poisoned the decision-making process and muzzled the voice of the people. Something radical needs to be done — and while this measure is only a small, measured step in the right direction, it’s an important and promising experiment. Prop. 15 would create a pilot public financing program for the 2014 and 2018 races for California Secretary of State — and the program would be funded by a tax on lobbyists. Right now lobbyists pay only $12.50 per year to register with the state. This measure would increase that fee to $350 annually and use the money to create a fund of about $6 million that candidates for the crucial office overseeing elections in the state could tap after demonstrating their popular support by gathering a number of small contributions. All candidates who qualify would be given the same amount of money and left to compete on the issues. Ideally this public financing program would prove successful and eventually be expanded to other offices. Public financing of election campaigns, which is currently working well in Arizona and Maine, is certainly worth a try in California. Vote yes.

7. Marriage Equality is on the Ballot

The crowded race for governor of Maine features a number of impressive candidates. But it is hard to fail to be impressed with the forthright approach of Maine State Senate Libby Mitchell, a contender for the Democratic nod, who has made the fight to restore Maine’s same-sex marriage protection central to her candidacy.

Says Mitchell:

I was raised a child of the segregated south where discrimination was just the way it was. My father ran a small grocery store, and it was watching him treat everyone that came in the store – whether they were black or white, rich or poor – with dignity and respect that formed my beliefs that all people are equal. I have fought all my life for the rights and protections of all Mainers.

This past session, I was privileged to help the State Senate pass Marriage Equality. During the debate, I laid down my gavel as Senate President to speak directly from the Senate floor on my views. In that speech I recalled the experiences of my childhood, addressed my colleagues respectfully, and made clear that the basic values of fairness and love were behind my vote in favor of equality in marriage. Many skeptics thought that the Senate would never be able to pass the bill, but under my watch we did. By listening to individual Senators and engaging them in the conversation, we made history.

While the bill was overturned in the referendum process, I still believe that Mainers will not be deterred from continuing to stand up for the rights of our friends, family, and neighbors. I will continue to work with advocates to make Maine a fairer and more just state.

And as Governor, I will work to make sure legislation for marriage equality reaches my desk, and I will sign it without hesitation.