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.