In the poll, Obama led Romney by 50 percent to 45 percent among Americans judged "likely" to vote by Peter Hart and Bill McInturff, who conducted the NBC/WSJ survey.

The Democratic incumbent also reached the 50 percent mark, to Romney's 44 percent, among the larger group of all registered voters.

The findings come at a challenging time for Romney's campaign. Two weeks before his first general election debate against Obama, and seven weeks before Election Day, the former Massachusetts governor faces backbiting within his campaign and finds himself on the defensive over secretly taped remarks at a Florida fundraiser. (Read More: Romney Derides Obama Supporters in Damaging Video.)

Obama benefited in the survey from an uptick in optimism over the economy as well as the general state of the country.

Some 39 percent of registered voters said the country is "headed in the right direction," up from 32 percent before the Republican and Democratic conventions. Some 42 percent predicted the economy will get better in the next year, while just 18 percent predicted it will get worse. In July, voters split evenly on the question. (Read More: Why Obama's Up in Swing States Despite Bad Economy.)

The shift marks "an important inflection point" in a race that has resisted movement for most of the year, said McInturff, a Republican pollster. Hart, a Democrat, ascribed the change to an increasing number of voters "getting comfortable with the next four years" of Obama in the White House.

"Barack Obama has moved a clear step ahead" in the race against Romney, Hart concluded. But he noted that "it's only a step" — and subsequent events could wipe out the president's advantage.

In the survey, Obama's overall job approval also hit the 50 percent mark, which political analysts generally consider an important sign of an incumbent's ability to win re-election.

Approval of his handling of the economy ticked up to 47 percent from 44 percent last month in tandem with rising optimism, while approval of his handling of foreign policy ticked down to 49 percent from 54 percent as anti-American protests spread across the Middle East in recent days. (Read More: Obama, Romney Brawl Over Embassy Attacks.)

The poll showed that Obama and his party have had success in shifting the campaign dialogue from a referendum on the state of the economy, Romney's preferred point of emphasis, to a choice about the future that benefits the Democratic ticket. By a narrow 41 percent to 38 percent margin, more voters called themselves worse off than when Obama took office rather than better off.

Yet when asked which candidate is better prepared to lead the country over the next four years, Obama enjoyed a double digit edge over Romney, 47 percent to 36 percent. The telephone survey of 900 registered voters, conducted Sept. 12-16, carries a margin for error of 3.27 percentage points.