Voters are going to the polls in 12 U.S. states to choose candidates in party primary elections.  Activists in both major political parties - the Democratic and the Republican Parties - have opened up many races, and political analysts say some upsets are likely.

In the south central state of Arkansas, two-term U.S. senator Blanche Lincoln faces a tough runoff against Bill Halter, the state's lieutenant governor.  They are seeking the Democratic Party nomination for the November election, and were forced into a runoff when neither received more than 50 percent of the vote in a primary vote last month.  

Lincoln has endorsements from President Barack Obama and former President Bill Clinton.  But labor unions have spent millions of dollars to unseat her.  They are angry over Lincoln's support of Republican-backed tax cuts and endorsement of positions that they consider to be anti-union.  

Challenger Halter says the two candidates disagree on many issues.

"There are just very profound differences between Senator Lincoln and myself," said Halter.

The winning candidate will face another tough election against Republican candidate John Boozman in the November general election.  Public opinion surveys show Boozman leading in a potential match-up against either Democrat.

Nevada's Republican governor, Jim Gibbons, has trailed badly in public opinion polls and could lose the nomination to Republican challenger Brian Sandoval.  If he does, it would be the first time in the western state's history that a sitting governor lost his seat in a primary election.

The Tea Party movement of libertarians and conservatives, who promote lower taxes and smaller government, has opened many races.

In Nevada, Republicans are battling for the chance to unseat U.S. Senate Majority Leader Harry Reid.  Tea Party favorite, Republican Sharron Angle has been leading in opinion polls against former state Republican chairwoman Sue Lowden and Las Vegas businessman Danny Tarkanian.  

Angle says she will phase out programs like the social security pension system.  And she wants the United States to withdraw from the United Nations.  Some Democrats hope she will win, saying that Angle poses less of a threat to Harry Reid in November.

California has high-profile races for governor and the U.S. senate.  Republican billionaire Meg Whitman, who once headed the Internet company eBay, has spent more than $70 million of her own money in the race for governor, battling the state's insurance commissioner, Steve Poizner.  The winner will face Democrat Jerry Brown, the state attorney general and former governor, in the November election.  

California's current governor, Arnold Schwarzenegger, cannot run again because of term limits.

Carly Fiorina, the former chief executive of computer giant Hewlett-Packard, is seeking the Republican nomination in the race against Democratic Senator Barbara Boxer.  Fiorina, who has positioned herself as a conservative, faces a former U.S. representative, moderate Tom Campbell, and conservative Chuck DeVore in a Republican contest that has highlighted conservative issues.  The winner will face a tough challenge in the general election against long-time liberal Democratic Senator Barbara Boxer.

Allegations of marital infidelity have livened up the governor's race in South Carolina, where a field of Republicans and Democrats is vying to replace Republican Governor Mark Sanford.  Sanford's widely publicized extra-marital affair with a woman in Argentina had already added spice to state politics.  Sanford has served eight years as governor and cannot run again because of term limits.

Republican Nikki Haley says she hopes to become the state's first woman governor.  Two men, including Larry Marchant, a former consultant to rival candidate, Lieutenant Governor Andre Bauer, claim they have had extra-marital affairs with Haley.  

"We had sexual relations.  We had sex," said Marchant.

Andre Bauer, who is accused of spreading the rumor, took a polygraph test to prove he did not.   Haley denies any infidelity and says this is just politics.  Her popularity surged after former Alaska governor and vice presidential candidate Sarah Palin endorsed her.