Just how severe is this year's anti-incumbent fever? Three Senate primaries on Tuesday will provide a clear gauge of the electorate's mood ahead of the crucial midterm election in November. Here, a quick guide to the races, and how to interprete the results:

PENNSYLVANIA - Democratic Senate primary
The candidates: Sen. Arlen Specter, 80, was first elected to the seat 30 years ago, and he won an endorsement from President Obama after switching parties last year. His opponent is Rep. Joe Sestak, 58, who has run ads saying, "It's time for a new generation of leadership."
What the polls say: Specter led early in the campaign by 20 percentage points. More recently, Sestak has pulled ahead.
What's at stake: A Specter loss would not look good for Obama, and would boost expectations of a general drubbing of incumbents — especially Democrats — in the fall.

ARKANSAS - Democratic Senate primary
The candidates: Sen. Blanche Lincoln, a moderate Democrat, is defending her vote for health care reform — but Arkansans overwhelmingly take dim view of ObamaCare. Her opponent, Lt. Gov. Bill Halter, says voters are so angry that Lincoln would be sure to lose in November's general election. "I clearly have a better shot," he says.
What the polls say: A recent poll shows Lincoln leading Halter 44 percent to 32 percent — but by state law she needs 50 percent of the vote to escape a June 8 run-off.
What's at stake: A win for Lincoln would be a boost to President Obama, who has taped ads for her. Regardless of the outcome, Republicans smell blood and are expecting to capture the seat in November.

KENTUCKY - Republican Senate primary
The candidates: Rand Paul is a founder of the Tea Party movement in Kentucky and the son of GOP libertarian and former presidential candidate Rep. Ron Paul. His main rival, Secretary of State Trey Grayson, was hand-picked by Senate Majority Leader Mitch McConnell and other leaders of the party.
What the polls say: Paul has surged to a double-digit lead in the polls.
What's at stake: The Kentucky race is a crucial test of Tea Partiers' power at the polls — Paul victories on Tuesday and in November would constitute a historic triumph for the grassroots anti-tax movement. A Grayson loss would be blow to the GOP establishment.

Sources: Los Angeles Times, MSNBC, New York Times