Seeking to build momentum heading into the Sept. 16 Democratic primary, state Sen. Dianne Wilkerson's challenger Sonia Chang-Diaz this week released an internal poll showing the veteran lawmaker down by 17.5 percent.

In a poll taken last week of 417 likely Democratic primary voters, Chang-Diaz outpolled Wilkerson by 47.2 percent to 29.7 percent. Twenty-three percent were undecided. The margin of error is 4.8 percent.

Chang-Diaz campaign officials said the results showed the effect of Chang-Diaz's aggressive door-knocking in the second Suffolk district, as well as the recent announcement of a settlement between Wilkerson and the state attorney general's office, in which Wilkerson pays a $10,000 for violating multiple campaign finance laws. She is also under a more scrutinized fundraising reporting schedule.

The results also come despite Wilkerson's deep support among other elected officials, including Gov. Deval Patrick, Senate President Therese Murray and Mayor Thomas Menino, according to Chang-Diaz campaign officials. Wilkerson's campaign declined to comment on the poll.

The poll also showed that 25 percent of voters believe Wilkerson deserves to be re-elected, with 46.9 percent believing someone else should take the Senate seat. The poll did not include any "message testing" or attempts to move anybody into either the Wilkerson or Chang-Diaz column, according to Dan Cohen from Connection Strategies, which executed the poll.

"It was not a push poll," he said.

Chang-Diaz, a former State House aide and teacher, came close to beating Wilkerson in 2006, when Wilkerson was unable to submit enough signatures to make the ballot and both ran sticker campaigns. With no Republican opponents to face in the November general election, whoever wins the re-match will cruise to taking office in January.