Democrats enjoy a slight advantage over their Republican challengers in three key Senate races in Ohio, Florida and Virginia, according to three new NBC-Marist polls released Thursday.

Democrats lead by varying margins in a series of contests that could determine whether Republicans are able to achieve the net-gain of four seats they need to retake control of the Senate if President Barack Obama wins re-election; they need to pick up just three seats if the Democratic incumbent loses.

A separate poll shows a virtually tied race in a fourth state, Massachusetts, that the GOP is battling to hold in November.

Obama edges Romney in 3 key battleground states

In Ohio, Democratic Sen. Sherrod Brown leads his Republican opponent, state Treasurer Josh Mandel, by a 14-point margin among registered voters. Fifty-one percent of registered voters said they would re-elect Brown if the election were held today, versus 37 percent who would choose Mandel; 12 percent were undecided.

In Virginia, former governor and Democratic National Committee Chairman Tim Kaine leads former Republican Sen. George Allen, 49 percent to 43 percent, among registered voters.

And in Florida, 46 percent of registered voters prefer Democratic Sen. Bill Nelson to 42 percent who said they would vote for Republican Rep. Connie Mack.

PDFs: Ohio | Virginia | Florida

The Florida, Ohio, and Virginia polls were conducted by Marist for NBC News between May 17-20. Each sample of registered voters has a 3 percent margin of error.

Democrats are tasked with defending 23 seats in this year's elections, many of which were won in 2006 in swing or Republican-leaning states. The three states in the NBC-Marist polls are also battleground states in the presidential election, and are expected to be heavily contested by both the Obama and Mitt Romney campaigns.

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Republicans argue that the variety of states where Democrats must play defense gives the GOP a number of paths toward capturing the majority, but the GOP's once-heady prospects for winning the upper chamber have been tempered by recruiting flameouts and some degree of Democratic resurgence.

The tightening battle for control of the Senate has also raised the pressure on Republicans to defend each of the seats they currently possess. For that reason, there's arguably no more highly-scrutinized Senate race than the one in Massachusetts, where Republican Scott Brown is working to hold onto his seat in a deeply-Democratic state.

A Suffolk University poll released late Wednesday night showed Brown's battle against Democratic challenger Elizabeth Warren, a favorite candidate of liberal activists, locked in a virtual tie.

Forty-eight percent of likely voters would elect Brown, who first won office in a Jan. 2010 special election to fill the seat of the late Democratic Sen. Edward Kennedy, to a full term. Forty-seven percent of Massachusetts voters said they would send Warren, a former Wall Street watchdog, to the Senate.

That race has been particularly hard-fought, involving most recently a controversy involving instances in Warren's career when she listed herself as having Native American heritage. But the Suffolk poll found that most voters in the Bay State didn't view the controversy as a significant story; likewise, Suffolk's numbers found that they didn't view a vote for Brown as akin to a vote for Wall Street.

The Suffolk poll, conducted between May 20-22, has a 4 percent margin of error.