With Democrats aiming to keep control of the U.S. Senate and Republicans hoping to wrest it away from them as well as hang onto their majority in the House of Representatives, four states will hold primary votes on Tuesday to choose their party's candidates to run for Congress in the Nov. 6 election.
Voters in Wisconsin and Connecticut will pick candidates for open seats being vacated by retiring Senators Herb Kohl and Joseph Lieberman. Kohl is a Democrat, and Lieberman is an independent but caucuses with the Democrats.
In Florida, the two-term Democratic Senator Bill Nelson is facing minor opposition in the primary but is expected to be in for a tough re-election battle in November against the likely winner of the Republican nominating contest.
Minnesota Senator Amy Klochubar, a Democrat, is expected to survive a primary challenge on Tuesday, while several Republican hopefuls are vying for the chance to run against her.
Democrats control the Senate by a 53-47 majority. Two years ago, Republicans seized control of the House and hold a 240 to 192 majority.
The Cook Political Report considers seven of the 23 Democratic-controlled Senate seats up for election this year to be toss-ups. Three of the 10 Republican-held seats up for election this year are toss-ups.
"It's a 50-50 ball game right now," said Cook Political Report analyst Jennifer Duffy. "When I look at the map, I find it improbable that any party would have 52 (Senate) seats, with 51 more probable."
A 50-50 tie in the Senate would give control of the chamber to the candidate who wins the presidency - Democratic President Barack Obama or his Republican challenger, former Massachusetts Governor Mitt Romney.
A wild card in the Senate will be if former Maine Governor Angus King captures, as expected, the seat of retiring Republican Olympia Snowe. King has said he will not declare which party he will side with until after the November vote.
In Connecticut, the favored Republican is Linda McMahon, a former professional wrestling executive who lost another Senate race two years ago despite spending $50 million. On the Democratic side, U.S. Representative Christopher Murphy is leading polls, and has already been targeted by McMahon's campaign ads.
In June, a Quinnipiac University poll found Murphy with a slight lead over McMahon if the two candidates were to win their primaries and face one another in the November general election.
Florida Republicans are expected to nominate U.S. Representative Connie Mack, who a recent poll showed could edge out Nelson in the general election. Mack's main primary rival is Dave Weldon, a former congressman and conservative Christian who has picked up a number of newspaper endorsements but has raised little money.
In Wisconsin, veteran former Governor Tommy Thompson and businessman Eric Hovde are expected to vie for the nod to face Democratic U.S. Representative Tammy Baldwin.
Whoever wins the primary may receive a lift from Romney's announcement over the weekend that he had chosen Wisconsin congressman Paul Ryan as his vice presidential running mate, analysts said.
Ryan is a polarizing figure in Washington, where he led his party's push to cut domestic spending, lower taxes and scale back the size of the federal government as chairman of the House of Representatives Budget Committee.
After a career as a Washington insider, Ryan may also be associated with the overwhelmingly negative view most Americans have of Congress. A Gallup poll last month found that only about 16 percent of Americans approve of the job Congress is doing.
"I think what the Democrats are trying to do with the Ryan pick is to run against the congressional Republicans," said Lee Miringoff, the director of the Marist Institute for Public Opinion at Marist College.
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