While the presidential campaign remains the main event of the 2012 election season, a fierce, state-by-state battle is also underway to determine which party will control the U.S. Senate for the next two years.  And a series of NBC News/Wall Street Journal/Marist polls shows Democrats with an edge in two of the most closely-watched races and tied in a third.

In Ohio and Florida, the Democratic candidate holds a solid lead, while in Virginia – where former governor and onetime DNC chairman Tim Kaine faces former Republican Sen. George Allen in a marquee battle – the candidates are in a dead heat.

Republicans had entered the 2012 cycle with high hopes of winning all three races. All are seats currently held by Democrats, and winning any of them would advance the GOP toward the net gain of four seats they need to take back control of the Senate come January.

In Virginia, Kaine and Allen are deadlocked at 46 percent apiece with likely voters.

In Ohio, incumbent Democratic Sen. Sherrod Brown leads Republican State Treasurer Josh Mandel 49 percent to 42 percent.

And in Florida, two-term Democratic Sen. Bill Nelson, leads GOP Rep. Connie Mack 51 percent to 37 percent. Twelve percent of Florida voters said they were undecided about the race.

With only 54 days until the election, these three polls suggest that Republicans still have work to do if they wish to achieve their goal of retaking control of the Senate. Democrats must defend 23 of the 33 Senate seats on the ballot this fall, a numerical disadvantage that buoyed GOP hopes of reaching their goal early in the cycle, especially since many of the Democratic-held seats are also in hotly contested presidential battlegrounds.

President Barack Obama leads Republican presidential nominee Mitt Romney in Virginia, Ohio and Florida, according to the same NBC News/Wall Street Journal/Marist polls released Thursday.

Each of the Marist polls was conducted Sept. 9-11. Each poll has a 3.1 percent margin of error for its sample of likely voters. 

Democrats control the Senate by a 53-47 majority. Two years ago, Republicans seized control of the House of Representatives in the 2010 mid-term election and hold a 240 to 192 majority.

In Florida, two-term Democratic Senator Bill Nelson faced minor opposition in his primary, but was expected to be in for a tough re-election battle in November against the Republican primary winner, U.S. Representative Connie Mack.

Mack, the son of a former senator, easily won the Republican primary over three other candidates and could edge out the incumbent Nelson in a general election, according to a recent poll. But political analysts said Nelson has ample resources to attack Mack.

"Tonight's results really show that a lot of Republicans are voting for the candidate they think will have the best chance of beating the Democrat" and putting aside negative concerns about individual candidates, said University of South Florida political analyst Susan MacManus.

Because of population shifts over the past decade, Florida added two congressional seats, but the redrawn districts pitted two incumbent Republicans against each other. Republican John Mica, a 20-year veteran, easily beat Sandy Adams, a favorite of the conservative Tea Party movement, in a central Florida district.

The Cook Political Report considers seven of the 23 Democratic-controlled U.S. Senate seats to be toss-ups. Nelson's re-election chances were seen as particularly tough. 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.

Wisconsin race 
A wild card in the Senate will be if former Maine Governor Angus King, an independent, wins 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.

Wisconsin and Connecticut voters set the stage to fill U.S. Senate seats being vacated by retiring Democrat Herb Kohl and Joseph Lieberman, an independent who caucuses with the Democrats.

Wisconsin Democrat Tammy Baldwin, a seven-term U.S. representative and avowed liberal, ran unopposed in her party's primary. Former four-term Wisconsin Governor Tommy Thompson beat businessman and political neophyte Eric Hovde and two other candidates for the Republican nomination.

Thompson may benefit in the general election from Romney's choice over the weekend of Wisconsin congressman Paul Ryan as his vice presidential running mate, analysts said.

However, 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.

The Connecticut contest was won by favorite Linda McMahon, a professional wrestling executive. McMahon is seeking another chance after she lost a Senate race two years ago despite spending $50 million.

On the Democratic side, U.S. Representative Christopher Murphy was favored to win the primary 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 face each other in the November general election.

Minnesota Senator Amy Klobuchar, a Democrat, was predicted to be heavily favored in November against the winner of the Republican contest. The party-endorsed candidate, Republican state representative Kurt Bills, was leading handily with more than half the vote counted. 

(Additional reporting by Edith Honan in New York, David Bailey in Minneapolis and David Adams, Tom Brown and Barbara Liston in Florida.; Writing by Andrew Stern. Editing by Cynthia Johnston and Christopher Wilson)