President Barack Obama has opened a slim lead over Mitt Romney in three battleground states, according to a new series of NBC News/Wall Street Journal/Marist poll.

Two of the states, Ohio and Florida, have long topped the list of battleground states where candidates of both parties invest significant time and resources. Virginia, which Obama won in 2008, has been gradually transforming into a swing state after decades of reliably falling into the Republican column.

In Florida (poll) and Virginia (poll), the polls found, Obama is leading Romney by a 5-point margin, 49 percent to 44 percent, which falls within the poll's three-point margin of error. In Ohio (poll), Obama has a larger lead of 50 percent to 43 percent. 

Those numbers capture likely voters. When the criteria are narrowed to Americans who are already registered to vote, Obama's advantage increased to 7 points in Virginia, 8 in Florida and 9 in Ohio.

Both campaigns and their allies have spent heavily on Ohio, Florida and Virginia -- a recent analysis of advertising spending by campaigns and independent advovacy groups found that the three states had attracted the most ad spending of any three states, accounting for more than $300 million between them.

The Romney campaign has sought to make this election a referendum on Obama's handling of the economy, and polls have shown voters giving the president low marks for his financial stewardship. But the new NBC News/Wall Street Journal polls found that voters in Florida and Virginia were split on which man would do a better job of overseeing the economy -- in Ohio, Obama has a four-point advantage.

A separate New York Times/NBC News poll gives Obama a three-point advantage nationally over Romney, although that edge is well within the poll's margin of error. The poll confirmed the candidates' respective areas of strength: Obama would carry female voters by a double-digit margin, while Romney captured men by eight percent.

The poll also found an income divide: Obama leads voters whose annual household income is under $50,000, while Romney enjoys a 16-point lead among voters with household incomes of more than $100,000.