The lead among registered voters represents a slight change from the NBC/WSJ poll in July, when Obama held a 49-43 lead -- suggesting Romney's campaign did not benefit significantly from the Aug. 11 selection of U.S. Rep Paul Ryan, R-Wis., as his running mate, NBC reported.
Obama leads Romney 94-0 among African-American voters, and has a 2-to-1 lead among Latinos. He leads 52-41 among voters under 35 years of age and 51-41 among women.
Whites prefer Romney 53-40 and he leads 47-38 among rural voters and 49-41 among seniors.
The two candidates are virtually tied among suburban, Midwestern and independent voters.
The poll found Obama leads Romney 49-46 in 12 battleground states -- Colorado, Florida, Iowa, Michigan, Nevada, New Hampshire, New Mexico, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin. Obama held an eight-point lead in those states in the July NBC/WSJ poll.
The poll results suggest Romney's two main challenges are repairing an unfavorable image with voters and persuading voters to support his plan to overhaul Medicare, NBC said.
Romney has a 38 percent favorable rating, compared to a 44 percent unfavorable score. Obama leads Romney by 35 points on the question of which candidate voters find more likable, and 51 percent of those surveyed said Romney was "out of step" with most Americans on issues, while 54 percent said Obama's positions are "in the mainstream."
Obama has a job approval rating of 48 percent, but only 44 percent of voters approve of his handling of the economy.
The poll of 1,000 registered voters was conducted Aug. 16-20. The margin of error is 3.1 percentage points.