A new Republican poll commissioned by YG Action Fund, a super PAC created by former aides to House Majority Leader Eric Cantor (R-Va.), found Republican Richard Tisei leading Rep. John Tierney (D) in Massachusetts’ 6th district.

The poll, in the field Sept. 25-26 and conducted by respected GOP firm North Star Opinion Research, found Tisei ahead of Tierney 45 percent to 37 percent. An August poll conducted by the same company found Tierney ahead of Tisei 45 percent to 43 percent. Negative advertising against both candidates has been plentiful in the district over the past month.

The polls did not include Libertarian candidate Daniel Fishman, who will be on the ballot in November.

In the September poll of the district, President Barack Obama led Mitt Romney by only 3 points, 48 percent to 45 percent. That’s probably a much slimmer  Obama lead than the actual presidential ballot number, Massachusetts insiders who have seen other private polls of the district told Roll Call.

In the September poll, Tierney had a favorable rating of 29 percent and an unfavorable rating of 47 percent. Tisei had a favorable rating of 33 percent and an unfavorable rating of 28 percent. Both candidates’ unfavorables have risen since the August survey.

The only public nonpartisan poll of the race, in the field earlier this month, found Tierney leading by 12 points and Fishman pulling 7 percent. In that poll, Obama led Romney by 10 points.

The Tierney campaign blasted the new GOP poll.

“Richard Tisei and his right-wing allies in Washington have been caught red-handed manipulating their poll data before, and it looks like they’re at it again here,” campaign manager Matt Robison said. “But nothing changes the fact that John Tierney is winning this race because he has an outstanding record of fighting for the middle class”

Beyond the specifics of the poll, the race — the only competitive Congressional contest in Massachusetts — is poised to be quite close and quite nasty.

Here’s the topline data from the poll, which surveyed 400 likely voters on land lines and cellphones using live telephone interviewers and had a margin of error of 4.9 points:

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YG Action Fund and its affiliated group have put almost $1 million into the Tossup race, including TV and other messaging. The mail pieces attacking Tierney — which have gone out to more than 170,000 households in the district, the group said — have been particularly nasty.

Here’s one of the YG Network mail pieces against:

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Here’s another:

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Democrats have also had hard-hitting direct mail out against Tisei. Here are two pieces the Massachusetts Democratic Party has sent to voters in the district:

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