Pennsylvania Sen. Arlen Specter said yesterday that his defection from the Republican Party was a matter of principle rather than opportunism, denied a report he had promised to be a "loyal Democrat," and vowed to join filibusters against his new party when he believes it is wrong.

"There's more than being reelected here - there's the factor of principle," Specter said on NBC's Meet the Press. "The Republican Party has gone far to the right since I joined it under Reagan's big tent. . . . As the picture has evolved, I felt a lot more comfortable - as a matter of principle - with Democrats than Republicans."

Specter also weighed in yesterday on the Supreme Court vacancy, saying he would like to see President Obama nominate a replacement for Supreme Court Justice David Souter who reflects "diversity," perhaps a woman, a Hispanic or an African American.

He said the president should look beyond the nation's appeals courts for candidates and might even be well advised to consider a non-lawyer. The Constitution does not require a justice to be a lawyer.

"I would like to see somebody with broader experience . . . somebody who has done something more than wear a black robe for most of their lives," Specter said on Face the Nation.

Specter, 79, stunned the political world Tuesday by announcing he was bolting the GOP after concluding that there was no way he could win the party's 2010 Senate primary against conservative former congressman Pat Toomey. Specter's standing among Republican voters plummeted after he joined Democrats to provide a crucial vote for Obama's $787 billion stimulus.

The new Democrat gave his first two broadcast interviews yesterday, and Sunday news shows across the dial featured discussion of the political fallout. Specter's decision has intensified the debate among Republicans about whether the party will be better off hewing closely to conservative principles or reaching out to attract and keep moderates.

Specter told Face the Nation's Bob Schieffer that it was also important that he remain in the Senate because he had become the "spear carrier" for increased federal funding on medical research.

"That research has saved or prolonged many lives, including mine," said Specter, a survivor of Hodgkin's lymphoma. "When you talk about life and death and medical research, that's a much more major consideration . . . contrasted with which party I belong to."

On Meet the Press, Specter flatly denied a report in the Wall Street Journal that quoted him telling Obama, "I'm a loyal Democrat. I support your agenda." He said, in effect, not to believe everything in newspapers and added that he would not hesitate to join 40 remaining Republicans to uphold a filibuster to block actions by the Democratic majority as his "conscience" dictated.

Specter said that Democrats offered no inducements to get him to switch and that he did not ask the president and other party leaders to clear the primary field for him. "I'm prepared to run in a contested primary, but I don't want to run against a stacked deck, like I would have had to against the Republican primary electorate," he said.

Host David Gregory challenged that answer, asking whether the agreement to keep his Senate seniority was an inducement. "No, that's an entitlement," Specter responded. "I've earned that seniority."

Rep. Joe Sestak (D., Pa.), meanwhile, said on CNN's State of the Union that he would not be dissuaded from considering a primary run because Obama, Gov. Rendell and other top Democrats were backing Specter.

"I'm kind of disappointed in the Democratic establishment in Washington, D.C.," Sestak said. "What I need to know is what he's running for. If he has the right answer, so be it. We move on . . . I'm not sure he's a Democrat yet."