Americans go to the polls in 12 states on Tuesday in another burst of primary voting expected to provide a telling glimpse of the public's mood ahead of this fall's midterm elections. Will Tea Party candidates beat out mainstream Republicans? Will Democrats running for re-election be swept aside by anti-incumbent fever? (Watch a CBS report about the expected impact of Tuesday's elections.) Here are five key races to watch:

1. Is Blanche Lincoln liberal enough? Moderate Democrat Sen. Blanche Lincoln (D-AR) is running against her state's lieutenant governor, Bill Halter, but she's also fighting powerful unions and other hard Left Democrats who say Lincoln's not liberal enough. Her perceived errors: She supported free trade and failed to defend the public option and other progressive proposals during the health-reform debate. This race could have implications for moderate Democrats, says Chris Cillizza in The Washington Post. "While Lincoln's career is on the line Tuesday, organized labor's influence is on the ballot as well."

2. How conservative is California? Three Republicans are vying for the right to challenge liberal Democratic Sen. Barbara Boxer in California and the real question is: How powerful is the Tea Party's influence? Former Hewlett-Packard CEO Carly Fiorina has pulled ahead of her rivals, including Tea Party favorite state Assemblyman Chuck Devore. Many people have dismissed Devore's chances — California's size and geography have made it hard for Tea Partiers to rally voters behind their choice — but an unexpectedly strong Devore showing would suggest that anger over government spending runs deeper than expected. Another key indicator will be the state's GOP gubernatorial primary, in which former eBay chief executive Meg Whitman, who opposes Arizona's tough immigration law, is engaged in an ugly battle with wealthy former businessman Steve Poizner, who supports it.

3. Will a Tea Partier challenge Harry Reid? On the Right, Nevada has its own showdown between a Tea Partier, Sharron Angle, and a mainstream GOP candidate, Sue Lowden, a former head of the state party, both of whom are vying to challenge Senate Majority Leader Harry Reid. Tea Partiers view Angle's candidacy "as a bellwether for national races in the midterm election," says Brian Calle in the Orange County, Calif., Register, so if she wins she'll energize the protest movement, and encourage "a harder rightward thrust for Republican candidates." 

4. How powerful is Palin? State Rep. Nikki Haley is still favored to win South Carolina's Republican gubernatorial primary, despite a conservative blogger's claim that he and the married Haley engaged in "inappropriate sexual contact" several years ago. Haley's unwavering dismissal of his claims is one reason many believe she's still out-polling her three rivals. Another reason may be Sarah Palin's endorsement. Haley's "Tea Party support and popular calls for smaller government seem to have drawn in the state's disenchanted voters," say Jay Newton-Small and Katy Steinmentz in Time, but it's unlikely she'll come away with 50 percent of the vote, so "it looks like the state will be headed for a scandal-mongering runoff."

5. Is Inglis insufficiently conservative? The GOP primary in South Carolina's Fourth Congressional District is a tough one for incumbent Rep. Bob Inglis, "a conservative who's proven not conservative enough for today's grassroots voters," according to Time. His challengers have tried to make the primary "a referendum on his 2008 vote to bail out the nation's banking industry," which made him a target for Tea Party groups. Inglis appears unlikely to avoid a run-off, so he's in trouble — because judging by the polls the pro-Inglis vote is smaller than the anti-Inglis vote shared by his challengers.

Sources: Huffington Post, Orange County Register, Washington Post, AP, Time