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.