Michigan Sen. Debbie Stabenow comfortably leads her Republican challenger who ran a racially-charged ad in his campaign, according to the new NBC News/Marist poll released Wednesday.

Stabenow, a two-term incumbent whom Republicans had believed was vulnerable this cycle in the economically-challenged state, leads former Rep. Pete Hoekstra by 21 points among registered voters, according to the new polling data.

If the election between Stabenow and Hoekstra were held today, 53 percent of registered voters said they would elect Stabenow to a third term, while 32 percent would support Hoekstra. Fifteen percent of Michigan voters said they were undecided.

The numbers, while coming at an early point in the campaign, reflect an uphill climb for Hoekstra, whom Republicans had hoped would offer their best shot to unseat Stabenow. A former committee chairman during his time in Congress, Hoekstra had fallen short in his gubernatorial bid in 2010 after losing in the Republican primary. But the GOP's success as a whole statewide that year had stoked optimism about their chances to beat Stabenow, a senior Senate Democrat and chairwoman of the chamber's agriculture committee.

Hoekstra's disadvantage may well reflect a degree of fallout related to an ad run by his campaign in Michigan on Super Bowl Sunday. The Republican candidate took fire for racial overtones in the ad, which depicts an Asian woman speaking in broken English, facetiously thanking Stabenow for spending policies which, the ad contends, help China.

Hoekstra's campaign, which had initially stood by the ad, has now scrubbed it from its YouTube page and has taken down a related website.

NBC News and Marist also tested a Senate matchup in the border state of Arizona. Republican Rep. Jeff Flake, a darling of fiscal conservatives, leads Democratic challenger Richard Carmona, the former U.S. Surgeon General.

Forty-two percent of registered Arizona voters said they would support Flake if the election were held today, versus 29 percent who would vote for Carmona; 28 percent of Arizonans said they were undecided.

Both Flake and Carmona are running to succeed retiring Sen. Jon Kyl, the No. 2 Senate Republican.

The polls of Arizona and Michigan were each conducted Feb. 19-20, The sample of registered voters in Michigan has a 1.8 percent margin of error, and the sample of Arizona registered voters has a 2 percent margin of error.