U.S. Republican presidential hopeful Newt Gingrich enjoys a double-digit lead over Mitt Romney, his closest rival in the GOP nomination race, but is much less competitive against President Barack Obama, according to a new national poll released Tuesday evening.

Forty percent of likely Republican voters support Gingrich, the former house speaker, as the party's nominee, compared with 23 percent who back Romney, the former Massachusetts governor, according to the NBC News/Wall Street Journal poll.

None of the other candidates, congressman Ron Paul, congresswoman Michele Bachmann, Texas Governor Rick Perry, former Utah Governor Jon Huntsman, and former Senator Rick Santorum, is in double-digits.

Riding strong performances at Republican primary debates, Gingrich recently rocketed in opinion polls, both nationally and in some key early voting states, to become the undisputed front-runner in the race for the GOP nomination.

While there have been front-runners at various moments in the GOP primary race to date, none of them seemed to have posed a serious threat to Romney as Gingrich does now, not only because the former speaker's increasingly wide lead in the field, but also the timing of his surge, less than a month away from the first GOP nominating contest in Iowa on Jan. 3.

Still, the Republican primary, the most volatile and unpredictable in decades, is far from settled. Many doubt whether Gingrich can sustain his recent momentum and translate it into real votes, given that despite his intellectual strength, he is widely considered unstable and undisciplined.

The survey also showed that Gingrich fares worse than Romney in head-on-head match-up against Obama.

Among all registered voters, Obama leads Gingrich by a substantial margin, 51 percent to 40 percent. Against Romney, Obama leads by just 2 points, 47 percent to 45 percent.

Gingrich has demonstrated his popularity among conservative voters, winning a significant boost to his campaign as conservatives typically are more enthusiastic in the GOP primary and have more influence on the race.

But surveys also showed his weakness among independents, an important voting bloc in general elections.