Stupak said his involvement in health reform legislation, rather, had led to a sense of "accomplishment," which led him this afternoon to announce his retirement at the end of this term.

"After 18 years, together we've accomplished what you sent me to Washington to do," Stupak said in a press conference in his northern Michigan district on Friday, referencing the healthcare bill that Democrats approved last month.

"My service to the people of Michigan has been one of the greatest honors of my life," the centrist Democrat added. "But it's time to begin a new and exciting chapter."

Stupak had drawn the enmity of both the left and the right for his role in health reform, having held out against voting for the bill until the president agreed to add restrictions on abortion rights in the package.

Liberals were angered by the abortion provisions, while conservatives accused Stupak, who opposes abortion rights, of enabling legislation which funds abortion services.

The congressman rejected the notion that the tough vote, and the threats against him and his family he'd received in its aftermath, had spurred his retirement.

"I'm not afraid to do tough votes," he said. "Those threats -- those three in the morning phone calls -- that's people outside the district. That's not my district."

The open seat in the rural, upstate Michigan district gives Republicans a prime pickup opportunity this fall, though Stupak dismissed the GOP contenders for his seat.

"I'm committed to help the Democrats retain this seat," he said. "Am I worried about it? Honestly, no. I've seen the Republican field, and I'm not impressed. There are many Democrats who can hold it."

Stupak had been wrestling with whether to retire for weeks.

A little more than a week before the healthcare bill was approved by the House, Stupak told The Hill that fighting the bill had been "a living hell."

He said phone lines to his office were jammed, and he and his wife received nasty calls at home.

“All the phones are unplugged at our house — tired of the obscene calls and threats. She won’t watch TV,” Stupak said during the interview. “People saying they’re going to spit on you and all this. That’s just not fun.” Stupak's wife had reported receiving death threats.

After the bill was approved, Stupak gave the Democrats' argument on the House floor against a GOP motion to kill the bill.

A Republican lawmaker, Rep. Randy NeugebauerRandy NeugebauerTop CFTC aide joins boutique K Street firm Yahoo hack spurs push for legislation It is time to abolish the Durbin Amendment MORE (Texas), shouted "baby killer" as Stupak was speaking. Neugebauer later apologized and said he was referring to the bill.