The poll gives Tisei 37 percent support to Tierney's 31 percent support, with 30 percent of those polled in Massachusetts' 6th District undecided. It's a near-reversal of the other most recent independent poll, which gave Tierney a seven-percentage-point lead over Tisei.

The new poll, conducted for the Boston Globe, indicates Tisei has more crossover appeal than Tierney, as he takes 10 percent of Democrats, while Tierney only takes five percent of Republicans. And slightly more than a third of Democrats remain undecided, meaning there may be room for Tisei to gain an advantage there if Tierney is unable to shore up his base.

Tierney faces a strong challenge from a Republican, even in a deep-blue state where President Obama leads by double digits, largely because of controversy surrounding his family's gambling ring. Though the congressman has said he knew nothing of the ring, his wife landed in jail for her role in the situation and Republicans have sought to raise questions about Tierney's reliability on the issue.

Though the poll doesn't address whether Tierney has been hurt particularly by the bad press about his family, it does show that his favorability rating is underwater, with 32 percent of voters viewing him favorably and 43 percent of voters viewing him unfavorably.

Tisei remains generally well-liked thus far, with 33 percent of respondents viewing him favorably to 22 percent viewing him unfavorably. Thirty-seven percent said they didn't know, however, leaving the door open for Tierney to define Tisei in the remaining weeks of the race.

However, Tierney communications director Grant Herring pushed back on the poll, pointing out that it skewed the margin of Democrats to Republicans in the district and omitted the Libertarian candidate, Daniel Fishman, who received six percent support in another recent poll.

"Overall, these polls are saying what we always knew all along: that his will be a very competitive race. John Tierney is going to win because voters understand that Congressman Tierney is committed to fighting for the middle class, while Richard Tisei supports the Tea Party agenda that will end Medicare and raise taxes on the middle class to pay for tax cuts for the wealthy," he said in an email to The Hill.

The poll was conducted by the University of New Hampshire Survey Center among 371 likely voters in Massachusetts' 6th district from Sept. 21-27 and has a margin of error of plus or minus 5.1 points.

--This post was published at 1:17 p.m. and updated at 2:23 p.m.