New polls released Thursday show Democrats ahead of Republicans in three big Senate races, all of which take place in crucial presidential election battleground states.

Starting with Ohio, incumbent Democratic Sen. Sherrod Brown enjoys a 14-point lead over Republican challenger and state treasurer Josh Mandel. According to the NBC-Marist survey, 51% of registered voters said they would support Brown, while 37% said they would pick Mandel.

The margin comes as good news for Ohio Democrats after they suffered major setbacks in 2010, losing the governor's office, a Senate seat and five U.S. House seats. The same poll shows President Barack Obama ahead of presumptive GOP nominee Mitt Romney in the state, 48%-42%.

Crossing over to Virginia, where two former governors are battling it out in one of the most closely-watched Senate races, Democrat Tim Kaine has moved in front of his GOP rival, George Allen. According to the poll, Kaine comes in with 49% versus Allen at 43%, a margin that equals the sampling error.

Kaine, who left his post as Democratic National Committee chairman last year to run for the Senate, has been statistically tied in other polls for months with Allen, who once held the same Senate seat he's now trying to recapture. The winner will replace retiring Sen. Jim Webb, a Democrat.

Republicans also made big gains in Virginia since the 2008 election. The GOP swept all statewide offices in 2009 and picked up the governor's office, as well as the Virginia state legislature, that same year.

But the latest NBC-Marist poll indicates Democrats may be picking up steam in the commonwealth. Vice President Joe Biden is injecting his voice in the race and will make an appearance at a private fundraiser for Kaine on Thursday.

In terms of the presidential race, Obama, who, in 2008, became the first Democrat to carry the state since 1964, pulls in front of Romney, 48%-44%, a margin within the poll's sampling error.

Looking at Florida, Democratic Sen. Bill Nelson appears to hold a slight advantage over Republican opponent Rep. Connie Mack. Forty-six percent of registered voters said they would re-elect Nelson, while 42% said they would chose Mack. The margin falls within the poll's sampling error.

Elected in 2000, Nelson is aiming for a third term. While he won re-election in 2006 by a landslide over GOP challenger Rep. Katherine Harris, his run this time around may be more competitive. Florida has seen a Republican surge since then, and Nelson is the only Democrat remaining in a statewide office.

In addition, the Republican National Convention will be held in Tampa this year, which will turn the political spotlight on the state's Republican Party.

Mack, a popular four-term Congressman, is the leading Republican running in a crowded field of candidates. The primary is set for August 14, just weeks before the convention.

The poll also shows Obama is ahead in the Sunshine State, with 48% to Romney's 44%. The difference falls within the sampling error, meaning the two candidates are statistically tied. Obama narrowly beat Republican Sen. John McCain in Florida in 2008, 51%-48%.

The NBC-Marist polls came one day after a separate poll showed a Democrat barely behind in the highly contested Massachusetts Senate race. According to a Suffolk University/7News survey released Wednesday, former Obama adviser Elizabeth Warren falls one point behind (47%-48%) incumbent Republican Sen. Scott Brown, who's fighting to keep his seat in the heavily Democratic state.

For the Ohio, Virginia and Florida surveys, 1,078 registered voters were surveyed in each state by telephone from May 17-May 20. All three polls have a sampling error of plus or minus three percentage points.