Three weeks and several million dollars in campaign ads later, Democratic voters in Arkansas head back to the polls Tuesday for the primary runoff that will decide the fate of Sen. Blanche Lincoln.

Farther west, Republican voters in California and Nevada will also pick their Senate nominees for two contests that are expected to be among the most expensive of the cycle. The three marquee Senate races are getting top billing Tuesday — which represents a Congressional Super Tuesday of sorts as 10 states are holding primaries.

Lincoln, seeking a third term, faces the fight of her political life against Lt. Gov. Bill Halter, who finished a surprisingly close 2 points behind her in a three-way contest May 18. Halter and the organized labor groups that have spent upward of $10 million on his campaign are hoping to finish the job Tuesday by defeating the moderate Senator, who has drawn the ire of liberals nationwide this cycle for opposing the Employee Free Choice Act and helping to kill the public option in the health care bill.

Late polling done for the liberal website Daily Kos, which is backing the lieutenant governor, showed Halter ahead by 4 points in the runoff, but that lead was within the survey’s margin of error.

A Lincoln defeat would be a major victory for the Service Employees International Union and other progressive groups that went out on a limb to back Halter in what was once seen as a long-shot bid. But it would also fuel the broader anti-incumbent storyline that has become a major theme of the 2010 cycle.

Some Democrats on Capitol Hill on Monday were already mentally preparing themselves for a Lincoln defeat, arguing that Halter would at least be able to play the outsider and anti-Washington cards in the general election against GOP nominee Rep. John Boozman.

But outside of each camp’s operatives, few Democratic insiders felt confident enough to call the contest one way or another Monday afternoon.

Throughout the runoff, Lincoln has hammered Halter for his union and labor connections, making the issue the focal point of a closing campaign ad that featured former President Bill Clinton.

“This is about using you and manipulating your votes,” Clinton said in the ad. “If you want to be Arkansas’ advocate, vote for somebody who will fight for you. Vote for Blanche Lincoln.”

Lincoln’s campaign manager also sent out a release over the weekend noting that union and labor-backed groups dropped about $2.5 million on TV ads for Halter during the runoff.

That release also accused those outside groups of campaign “hijinks” such as distributing a door hanger with a photo of Halter superimposed next to President Barack Obama in African-American neighborhoods. Obama has endorsed Lincoln.

Halter campaign spokesman Bud Jackson described the Lincoln complaints over union involvement as “highly hypocritical.”

“People know she’s been backed to the tunes of millions of dollars from special interests, whether it’s the Chamber of Commerce or from Wall Street,” Jackson said.

To win tonight, Lincoln will need another strong performance out of Pulaski County, which contains the city of Little Rock, where she did unexpectedly well last month by beating Halter by 12 points.

Her supporters are hoping that Democratic runoffs in the Little Rock-based 2nd district as well as the 1st district — which Lincoln previously represented for two terms in the House — will boost turnout that will benefit the Senator.

But while Halter underachieved in Pulaski County last time, he exceeded expectations in some more conservative Democratic areas, especially in the 4th district. There’s no House Democratic runoff in the 4th to pull more voters out to the polls, but Halter has been working that area hard over the last three weeks.

“A lot of people made the false assumption that Bill is the liberal, Little Rock candidate,” one Halter campaign operative said Monday. “We did well among more conservative folks in those more rural areas. We’ll definitely be looking at those areas [again] tomorrow.”

One Arkansas Democratic consultant, who isn’t involved in the Senate race but is personally supporting Lincoln, gave Halter’s campaign credit Monday for an especially strong grass-roots and voter contact operation.

“They are working it in ways we haven’t seen ever before,” the consultant said. “Anybody that voted in the 2006 [lieutenant governor] runoff I think got a personal letter from the Halter campaign.”

Meanwhile, the consultant said Lincoln is relying on larger-scale events and more traditional avenues to get out the vote. Some campaign operatives are projecting turnout to be about 60 percent of the May 18 vote.

“It’s grass roots versus grass tops,” the consultant said.

But amid all the hype over special interest groups, campaign tricks and who has the better field operation, the runoff may be a simple referendum on Lincoln and whether the Agriculture Committee chairman can continue to serve her state well while walking a moderate line.

A woman who answered the phone during the lunch rush hour at Doe’s Eat Place — a gathering spot for politicos in Little Rock going back to the Clinton years — said she was still on the fence, but she seemed to indicate the contest was more about Lincoln than Halter or his union connections.

“She holds the Agriculture seat, but she also voted against the health care bill,” the woman said.

Lincoln’s defeat — or victory — will likely lead Wednesday’s national headlines, but hotly contested Senate primaries in two other states are also worth watching for clues about November.

In the race to take on Senate Majority Leader Harry Reid in Nevada, former state Assemblywoman Sharron Angle (R) has surged in recent weeks and has suddenly found herself ahead by comfortable margins in late polling.

Angle’s top rival now appears to be businessman Danny Tarkanian, after former state party Chairwoman Sue Lowden, the establishment pick in the race, saw her campaign derailed by a series of early stumbles and targeted attacks from Democrats and her Republican opponents.

An Angle victory will be celebrated by many Democrats, who believe her tea party connections and extremely conservative views make her an easier general election opponent for Reid.

In California, former Hewlett-Packard CEO Carly Fiorina will face three-term Sen. Barbara Boxer (D) if she performs as polls expect and comfortably wins a three-candidate GOP primary that includes former Rep. Tom Campbell and state Assemblyman Chuck DeVore.

Campbell, a moderate on social issues, led in early polls in part because of the name recognition he achieved during losing campaigns for the Senate in 1992 and 2000, and his primary-eve message has focused almost exclusively on his electability.

But Fiorina overtook him in the waning weeks of the campaign, in large part because of a robust television advertising campaign that touted her conservative credentials and was funded in part by her substantial personal wealth.

Boxer’s campaign acknowledges that Fiorina will be a tough opponent because of her fundraising strength and a flagging state economy that has weakened the traditional advantages of incumbency. In recent weeks, Boxer has enlisted Obama for fundraising help.

Greg Giroux contributed to this report.