The 4th district should be among this year’s top sleepers. At least that’s the storyline being pushed by Republican Sean Bielat, who is trying to knock off House Financial Services Chairman Barney Frank.

The Bielat campaign released an internal poll Wednesday showing Frank’s support “has dipped below 50 percent.” Bielat trails 48 percent to 38 percent, according to the survey of 400 likely voters by the Republican firm OnMessage Inc. The margin of error is 4.9 points.

As further evidence that the race is tightening, Bielat cites an upcoming visit by President Bill Clinton, who will headline a Frank campaign rally in Taunton on Sunday. The event, to be held at a local high school, is free and open to the public. Roughly 10,000 people are expected to attend.

“People are sick and tired of Washington and career politicians. That’s why Barney Frank’s poll numbers are dropping like a rock and why he’s taking desperate measures like bringing in Bill Clinton to campaign for him,” Bielat said. “National support is snowballing and our campaign and supporters are more enthusiastic than ever. It shows what we’ve known all along: We are on track to replace an entrenched politician who’s been part of the problem for years.”

The Frank campaign dismisses the Bielat polling numbers and the suggestion that the incumbent is in trouble.

“This guy is as loose with the facts as Sarah Palin is loose with intelligence,” a campaign aide said.

While nothing is impossible this year, a Bielat upset would be a shocker. The 4th district gave President Barack Obama 63 percent of the vote in 2008 and Sen. John Kerry (D-Mass.) 65 percent of the vote in the 2004 presidential race. Bielat reported just more than $84,000 in his campaign account as of Aug. 25, compared with Frank’s $1.02 million.