WASHINGTON — Arlen Specter, Pennsylvania's longest-serving senator, is expressing few personal regrets as he leaves office, and insists his decision to switch parties before losing in the Democratic primary was based on sound judgment.

Specter, a fixture in American politics and a centrist who often drew the ire of both Republicans and Democrats, spoke to reporters in his basement "hideaway" office Thursday for the last time.

Asked what he would miss about the job he has held for 30 years, he said, "It's time to move on." Asked what he would miss most, if anything, he gave the same answer, but added that he has some unfinished business.

"I'd like to be able to push the bill I have on embryonic stem cell research, I'd like to be a part of the appropriations process on NIH," he said, a reference to his support for medical research at the National Institutes of Health.

The Senate ended its two-year session Wednesday.

Specter said he will write his third book, pursue a career as a commentator, teach a class at the University of Pennsylvania on Supreme Court confirmation hearings — he has gone through 14, he said — and possibly practice law.

The 80-year-old former Philadelphia district attorney and architect of the "single-bullet theory" of the Kennedy assassination, declined to reflect on his emotions the night in May he lost his bid for a sixth Senate term.

Rather, he repeated his defense of the vote he cast in favor of President Barack Obama's economic stimulus plan as a bulwark against the country repeating the Depression he had lived through as a boy in Kansas. The vote angered Republicans, and Specter concluded that he would stand a better chance in a Democratic primary, with Obama's support, than in a Republican primary.

"In retrospect when the primary was lost and therefore the seat was lost, I thought about the decisions I've made and I reconstructed them one-by-one. They were all sound at the time, but they didn't work out," Specter said.

Specter lost the Democratic primary to U.S. Rep. Joe Sestak after switching his party registration in April 2009. Specter concluded that he would have faced a tough Republican primary challenge from former U.S. Rep. Pat Toomey, who went on to beat Sestak in the November general election.