A primer on some of the most important battles
Reshma Saujani vs. Carolyn Maloney (New York House - D)
First elected to Congress in 1992, Carolyn Maloney, 64, has represented New York City's 14th congressional district for nine terms. She is a senior Democrat on the House Financial Services Committee and is chairwoman of the Joint Economic Committee. Maloney won her 2008 election with 80 percent of the vote, facing no primary challenge. In 2008, Maloney voted for the $700 billion bailout of large financial firms and since then has been a persistent critic of Wall Street. She co-sponsored last year's law that required credit-card issuers to provide consumers with more detailed information about changes to their accounts. She also has been an advocate for women's issues and improved medical treatment for people who worked at Ground Zero after September 11.
Alan Mollohan vs. Mike Oliverio (West Virginia House - D)
Mollohan has spent much of his 28-year House career steering federal funds back home. That isn't the résumé-builder it used to be, especially in light of repeated exposés about his close relationship with the recipients. Opponent Oliverio, a state senator, chides Mollohan for those cozy ties and for his muted opposition to cap-and-trade legislation because it could harm the coal-mining industry.
What it Means: A Mollohan loss in the May 11 Democratic primary could signal a strong anti-incumbency mood.
Trey Grayson vs. Rand Paul (Kentucky Senate - R)
Senate GOP Leader Mitch McConnell, the godfather of Kentucky politics, endorsed Grayson, the secretary of state, to replace retiring Senator Jim Bunning. That might not be enough to repel Paul and his Tea Party supporters, including Sarah Palin. The May 18 GOP primary has become a proxy fight over the party's direction. Paul is the son of Ron Paul, the onetime Presidential candidate and current House member from Texas.
What it Means: Tea Party activists would send a powerful message to Establishment Republicans if Paul wins.
Blanche Lincoln vs. Bill Halter (Arkansas Senate - D)
Lieutenant Governor Halter is harnessing liberal dissatisfaction with the two-term Lincoln among unions and other Democratic primary voters. Lincoln recently tried to bolster her Main Street credentials by authoring legislation that would prevent big banks from conducting derivatives trading. If she beats Halter in the May 18 primary, her reward will be an equally bruising general-election fight.
What it Means: The race will test the ability of the angry liberal wing of the Democratic Party to replace moderates with one of their own in a swing state.
John McCain vs. J.D. Hayworth (Arizona Senate - R)
Radio host and former Representative J.D. Hayworth pitches himself as "The Consistent Conservative" in his bid to deny McCain a fifth term, forcing the senator to shun his "maverick" label and take a harder line on immigration. McCain accuses Hayworth of being a pork-barrel spender during his time in Congress.
What it Means: If McCain loses the Aug. 24 primary, even conservative Republicans who occasionally buck the party could have a hard time.