More power to Joe Sestak. No, that’s not an endorsement of the Pennsylvania Congressman, who, against the wishes of the Obama White House and Democratic strategists in Washington and Harrisburg, is mounting a primary challenge to Republican-turned-Democrat Arlen Specter. It’s an endorsement of primaries.

Primaries can be divisive and expensive. But they also bring clarity and needed attention to policy debates and generate effective and electable fall candidates. Don’t forget that the late Edward Kennedy was elected to the Senate only after winning an intense 1962 Democratic primary–or that Kennedy’s likely successor will be chosen in what’s shaping up to be a rip-roaring “special” Democratic primary this fall. Don’t forget that Barbara Boxer made it to the Senate after beating California’s lieutenant governor and a senior Congressman in a 1992 Democratic primary; that Russ Feingold was a surprise winner of a Wisconsin Democratic primary that same year; and that Daniel Patrick Moynihan, Claiborne Pell, Howard Metzenbaum, Paul Wellstone and even Barack Obama won competitive, at times bruising, primaries before becoming senators. Winners of hard-fought primary contests go into general-election campaigns with confidence, and if they have beaten the party establishment they are freed to run on their own merits–a status that helps attract independent votes, which are likely to be up for grabs in 2010.

So it’s good that Sestak is holding Arlen Specter to account for his cooperation with the Bush/Cheney administration on judicial appointments and the Iraq War–and that he’s pressing Specter for failing to take progressive positions on worker rights and trade policy. It is good that labor activist Jonathan Tasini and county legislator Jon Cooper are mounting intraparty challenges to New York Senator Kirsten Gillibrand, and that even more Democrats have talked of doing so in a process that has forced this former Blue Dog to adopt a more progressive stance.

It is good that the field of candidates to replace Illinois Senator Roland Burris–who, like Gillibrand, was appointed rather than elected–is attracting people like Chicago’s former Inspector General David Hoffman, a clean-government champion; Chicago Urban League president Cheryle Jackson; and President Obama’s basketball buddy State Treasurer Alexi Giannoulias. It is good that Los Angeles progressive activist Marcy Winograd is challenging Representative Jane Harman, the California Democrat whose coziness with the intelligence establishment has put her at odds with civil libertarians and whose economic policies echo those of corporate-friendly New Democrats, and that Democratic House and Senate primaries are shaping up in states across the country.

And it is good, indeed, that Illinois legislator Julie Hamos (the only other elected official to appear with State Senator Barack Obama at that famous 2002 antiwar rally in Chicago) and 2008 Democratic nominee Dan Seals are among those competing for the Democratic nod to grab a Republican-held Congressional seat representing the Illinois district that gave Obama’s presidential candidacy 61 percent of the vote.

It is unfortunate that President Obama, Vice President Biden and key players in the Democratic Congressional leadership continue to discourage contests–especially those that threaten vulnerable incumbents. The theory is that Democrats need to be unified for a fall fight against a Republican Party gunning for a 1994-style “revolution” that will put the brakes on the initiatives of a first-term Democratic president. After Specter switched parties, Obama, Biden and Pennsylvania Governor Ed Rendell jumped to endorse the former Republican, even as Specter refused to side with Democrats on issues like the Employee Free Choice Act.

Similarly, after New York Governor David Paterson’s surprise selection of Gillibrand to replace Hillary Clinton, the White House worked with New York’s senior senator, Chuck Schumer (the winner of a competitive Democratic Senate primary in 1998), to muscle prominent potential challengers out of the running. Bronx Representative José Serrano complained about “the White House getting involved in sort of the old Tammany Hall way” and griped that party leaders should “just leave us alone in New York to work it out.”

Serrano’s right. No doubt, the concern about resurgent Republicans is legitimate; the Party of No is recruiting competitive candidates (including relative moderates like former Hewlett-Packard CEO Carly Fiorina, a likely challenger to California’s Boxer) and beginning to raise money at a serious clip. But the fantasy that Democratic “unity” will preserve party majorities in Congress–especially if the party maintains a murky centrism on issues like healthcare reform, bank bailouts, unemployment and Afghanistan–goes against logic and history.

Democrats who want Obama to succeed should recognize that their party is usually at its best when it trusts grassroots activists and voters to make choices. Barely two years ago, much of the Democratic establishment settled on Hillary Clinton as the party’s 2008 presidential nominee. But a freshman senator challenged the conventional wisdom. He bet he could build a movement capable of winning primaries and rewrite the rules of American politics. Candidate Obama placed his faith in primary voters to choose a candidate who was in tune with what November 2008 voters wanted and needed from a Democratic nominee. President Obama and his aides should re-embrace that wisdom and let Democratic voters in Pennsylvania, New York, California and other states pick candidates who are in tune with what November 2010 voters want and need from Democratic nominees for the House and Senate.