Some 170,000 voters cast a ballot in early voting, roughly one-third the number that appeared during the one-stop period for the presidential primary two years ago. There's been no sign of a tea party surge, as Republicans have comprised just 33 percent of voters so far -- similar to last election and to the statewide breakdown of party registration.

"It's disappointing that more people are not tuning into the importance of the primary election," said Bob Hall, executive director of elections watchdog Democracy North Carolina. "It's the place where candidates are filtered out. The vote actually has tremendous weight."

At the top of the ballot, the election will weed out a large group of Democrats seeking to challenge Republican Sen. Richard Burr, whose approval numbers have sagged along with the rest of Congress. Burr himself faces GOP challengers who question whether he can get re-elected, though the incumbent is widely seen as a favorite and plans to spend Election Day in Washington..

There are also several key House primaries: Republican Rep. Howard Coble, who hasn't seen a primary challenger in a quarter century, faces several looking to represent his district in central North Carolina. Democratic Rep. Larry Kissell, who won a seat just two years ago to represent southern North Carolina, faces a challenge from one of his former campaign volunteers. Meanwhile, several Republicans have been competing in a costly race to challenge Kissell in the state's most competitive district.

Scores of other primaries across North Carolina ballots will help select state lawmakers, judges and prosecutors.

North Carolinians have expressed discontent with elected officials and particularly Congress; an Elon University poll released two weeks ago showed 69 percent of state adults said they disapproved of the way Congress was doing its job. That has yet to translate into mobilization at the ballot box, however.

The 170,000 voters who went to one-stop voting sites was higher than the 70,000 who turned out during the primary four years ago, but that election didn't have any major statewide decisions such as this year's U.S. Senate race. The election two years ago included both a presidential and Senate race and drew some 480,000 voters to one-stop sites before Election Day.

Gary Bartlett, the executive director of the State Board of Elections, said officials were hoping to see more early voters.

"I am hoping for a large turnout (Tuesday)," he said. "However, all the signs during my tenure lead me to believe it will not be a large turnout. I don't think we'll reach 20 percent."

Non-presidential primaries typically have voter turnouts around 20 percent.

Democratic U.S. Senate candidate Ken Lewis said Monday he's been deliberate during his campaign to have more personal engagement with the public, knocking on the doors of homes in Wilson and Chapel Hill in the campaign's final days.

"Our way is to reach out to voters," Lewis said in a phone interview, but "as a citizen, I certainly liked to have seen more people come out and vote."

The three leading candidates in that race -- Lewis, Secretary of State Elaine Marshall and former state Sen. Cal Cunningham -- scheduled visits Monday to their campaign phone banks in a sign that the low early-voting turnout means getting their supporters to the polls Tuesday is an even greater portion of the formula for a primary victory or entering a runoff.

A candidate must win more than 40 percent of the vote to avoid a two-person runoff.

Bartlett said elections investigators were looking into accusations of vote buying in Bladen County and an allegation that a candidate in eastern North Carolina provided something of value for a vote. He said it was common to hear accusations of impropriety shortly before Election Day.