Bielat joins a field of candidates that has grown larger and larger in the six weeks since Frank announced his retirement. At least four Democrats have made public their interest in the seat, including Joseph P. Kennedy III, the grandson of former presidential candidate and Sen. Robert Kennedy (D-N.Y.).

Paul Heroux, a businessman and policy consultant from Attleboro, Mass., on Tuesday became the latest Democrat to officially enter the race, calling himself an "independent Democrat" and portraying his candidacy as a rejection of the notion that only millionaires or the elite can successfully run for Congress.

On the Republican side, Bielat will face physician Elizabeth Childs, who had already entered the race before Frank announced his retirement last year.

Democrats greeted Bielat's official entrance into the race by painting him as a carpetbagger, claiming he had left for Pennsylvania after Massachusetts voters refused to give him a seat in the delegation.

"Now it looks like he has moved back to Massachusetts – again – to run the same failed campaign based on the same extremist Tea Party ideas," Massachusetts Democratic Party Chair John Walsh said in a statement. "Groundhog Day has come early to Massachusetts."