Although he is among the most vulnerable Democratic incumbents running for re-election this year, Sen. Mark Pryor has so far been holding his own against his Republican challenger, Rep. Tom Cotton, in public polling -- and the gap lately appears to be widening.

A new NBC News/Marist poll shows Pryor leading Cotton by 11 points, with 51 percent to Cotton's 40 percent. Notably, Pryor leads Cotton by a double-digit margin among women, and by seven points among independents.

“So far, Sen. Pryor has staved off Cotton's challenge,” says Dr. Lee Miringoff, Director of The Marist College Institute for Public Opinion. “Holding this seat is no easy task for a Democrat with President Obama's approval rating at 33 percent in the state.”

The results in this latest survey, published Monday, track approximately with a recent New York Times/Kaiser poll, which found Pryor leading Cotton by 10 points.

The trend indicates there is durable momentum for Pryor, who narrowly trailed Cotton in most public polls at the beginning of this year, when Democratic candidates in battleground states struggled to contend with low approval ratings for the president and Obamacare.

Cotton has lately attempted to recoup some support with a spate of positive biography television ads, including one featuring his father and the family's farm, and Cotton last month proposed a series of five debates with Pryor.

Pryor, for his part, felt confident enough to appear publicly this week with President Obama as he toured tornado damage in Arkansas. Other vulnerable Democratic incumbents, such as Sens. Mary Landrieu of Louisiana and Kay Hagan of North Carolina, have dodged public appearances with the president when he has visited their home states recently, but Pryor actually invited the president to tour tornado-ravaged areas.