Though Arkansas Senator Blanche Lincoln got more votes in last night’s Democratic primary, her opponent, Lieutenant Governor Bill Halter, was the night’s big winner, keeping Lincoln under 50 percent (she led 44-42 or by 9,000 votes) and forcing a runoff on June 8.

The other big winner, though he won’t be making it to round two, was right-wing businessman D.C. Morrison, who pulled an unexpected 13 percent of the vote. Morrison ran as a very, very conservative Democrat; he called global warming a “hoax,” threatened to filibuster the nomination of any “pro-abortion” judge to the Supreme Court and proudly noted that he voted for Ron Paul in 2008. Which way his supporters will go in the runoff—or if they’ll vote at all—is now the $64,000 question in Arkansas politics.

Conventional wisdom holds that Morrison voters, however inchoate, will gravitate towards Lincoln, since she’s more conservative than Halter. But Morrison voters also tend to be of the anti-incumbent, Tea Party, damn-Washington-to-all-hell variety, which makes it hard to believe they’ll support a longtime incumbent senator. Halter has been making a populist pitch to these voters throughout his campaign, saying at his election night celebration (which had all the buzz of a victory party), “If you believe that Washington works for you, then Senator Lincoln should be your choice.”

But a good chunk of Arkansans were fed up with both the Lincoln and Halter campaigns, especially as the race became infused with negativity (with Lincoln’s side widely considered the greater offender) and issues began to disappear from the debate. “End it, don’t extend it,” Arkansas Times editor Max Brantley urged.

Well, now it’s extended and Halter has a choice to make—if he wants to defeat Lincoln, rather than just give her a good scare, he’ll need to add some meat to his message, drawing a clear contrast between his campaign and Lincoln’s on style and substance (he’s done the former but not enough of the latter), as Joe Sestak did to Arlen Specter. Halter, in his speech last night, railed against “the special interests, the Washington, DC, insiders, Republican shadow groups, and the corporative executives and lobbyists who peddle greed over people from their Wall Street headquarters and Washington, DC, K Street offices.” But he offered no specifics about what he’d do differently from Senator Lincoln or why he’d be a stronger challenger against GOP nominee John Boozman, a three-term congressman from Northwest Arkansas who will be a strong candidate in a general election but will also face some of the same anti-incumbent angst currently directed towards Lincoln.

Lincoln remains vulnerable—she tacked right in Washington for much of the Bush and Obama years and then swerved to the left as Halter’s primary bid intensified. Halter can’t let her sudden conversion to progressive populism go unchallenged.

No matter what happens from here, the primary has been a good thing, forcing Lincoln to unveil a tougher derivatives bill than previously expected and energizing the progressive groups who’ve endorsed Halter, spurring them to organize effectively on politically conservative terrain. But, as last night showed, the newly ascendant progressive insurgents don’t just want to send a message to washed-up Washington insiders. They want to win.

UPDATE: A day after poll by Research 2000 shows Halter leading Lincoln by two points, for the first time, which would be a major momentum bounce if other polls confirm that finding in the next few days. Also, Max Brantley notes that Lincoln handily beat Halter in liberal Pulaski County (home to Little Rock) but lost big in the state’s rural areas, which is the exact opposite of what political insiders were predicting. That means Halter got a lot of votes from conservative Democrats, which runs counter to the media’s description of him as a fringe lefty. One labor source who’s backing Halter told me from Little Rock this morning that way more of the Morrison vote is projected to go to Halter than Lincoln. based on their polling. "DC media is playing this as a left-right fight, and therefore wants to put each candidates’ supporters in those neat little boxes, but that’s not how AR voters are viewing it," he wrote. "It’s way more ‘change versus more of the same.’ If we keep it that way we win."