Halter has the support of liberal activists such as MoveOn, League of Conservation Voters, and unions. The US Chamber of Commerce has endorsed Lincoln. Former President Bill Clinton and President Obama have also cut radio ads for Lincoln.
Eight Republicans are vying Tuesday for the right to challenge Lincoln or Halter in the November general election. A runoff is likely for that contest, too.
State Sen. Gilbert Baker, a former music professor, has created a grass-roots base from the anti-establishment side. Former state Sen. Jim Holt, who ran for Senate against Lincoln in 2006 and garnered 44 percent, is also considered a formidable candidate.
Recent polls show five Republican candidates beating Lincoln or Halter in a November match-up.
Lincoln and Halter have battered each other during the primary with television ads and mailers, especially about outsourcing jobs.