As Republican Richard ­Tisei prepared to present Democrats who are backing his campaign for Congress, a new poll shows his opponent, US Representative John Tierney, leading Tisei by seven points.

The WBUR poll, conducted by the MassINC Polling Group, showed 22 percent of the 401 likely voters surveyed were undecided, but 39 percent backed Tierney, compared to 32 percent for Tisei.

Tisei plans to hold a press conference Thursday to introduce Democrats supporting him, while Tierney has invited reporters to join a conference call Thursday and hear US Representative Barney Frank discuss Tierney’s work on ­behalf of the lesbian, gay, ­bisexual, and transgender community, Tierney’s campaign said.

On Wednesday, Tierney ran his first television ad of the campaign, a spot in which he links Tisei to Tea Party Republicans and highlights a report in which Tisei described the Tea Party as a “Godsend.”

Tisei’s campaign said the ad was “full of untruths.”

According to the National Republican Congressional committee, in 2010 GOP ­gubernatorial candidate Charles Baker and Tisei, his running mate, won the Sixth District by nine points.