With the Democratic presidential nomination lost to Senator Obama, backers of Senator Clinton are making a concerted and newly invigorated effort to win her the second spot on the ticket this fall, even though it is far from clear whether Mr. Obama or Mrs. Clinton would embrace such a pairing.

While some are touting Mrs. Clinton as a possible Senate majority leader, governor of New York, or even Supreme Court justice, the discussion of making the former first lady the vice presidential nominee has intensified substantially in recent days.

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A former chief of staff to President Clinton, Leon Panetta, added his voice yesterday to those urging an Obama-Clinton slate and even suggested that Mr. Obama’s willingness to accept Mrs. Clinton as a vice presidential nominee may reflect on his suitability for the Oval Office.

“It may be his first test as to whether or not he’s prepared to be president of the United States. If you want to be president, you’re going to have to be flexible in terms of what the most effective way is to get your message across to the American people,” Mr. Panetta told The New York Sun.

Conventional wisdom dictates that a presidential candidate avoid choosing a running mate who might steal the spotlight, but Mr. Panetta said Mr. Obama should set those concerns aside. “From my point of view, what it would show me is a sense of confidence on his part that he knows who he is and is not afraid of having that kind of personality as part of his team,” the former congressman and budget director said.

One of Mrs. Clinton’s early supporters on Capitol Hill, Senator Feinstein of California, made clear yesterday that she enthusiastically favors a so-called unity ticket, even as she nudged her candidate to exit the presidential race.

“I’m a very strong supporter of Hillary being placed on the ticket as a vice presidential candidate,” Mrs. Feinstein told CNN. “The two of them together, Barack Obama and Hillary Clinton, cover the constituencies of America and I think represent a working team that is unprecedented in the United States.”

In a conference call with members of Congress from New York yesterday, Mrs. Clinton reportedly signaled a willingness to accept the vice presidential slot if asked. One lawmaker, Rep. Brian Higgins of Buffalo, told the Buffalo News that the former first lady raised the issue. However, three other lawmakers on the call told The New York Sun that Mrs. Clinton suggested she was open to the idea only after it was floated by Rep. Nydia Velazquez of New York City.

Some hard-core Obama supporters in the blogosphere reacted negatively to the renewed talk of bringing Mrs. Clinton on board, arguing that she would undercut Mr. Obama’s ability to offer a clean break with the past. “Clinton is not acceptable under any circumstance,” one Obama backer wrote on the DailyKos site yesterday. “ABH — Anybody But Hillary.”

More staunch opposition is expected from Mr. Obama’s advisers, who are eager to run their campaign without unwelcome advice and intrusions from Mr. Clinton and the coterie of Clinton aides who would be part and parcel of adding the former first lady to the ticket. “I would still be stunned beyond belief if she were chosen,” a political analyst with a conservative-leaning think tank, Norman Ornstein of the American Enterprise Institute, said. “She motivates Republicans to turn out and vote in a way John McCain does not. There are also governing considerations. … The idea of a White House with Hillary Clinton and Bill Clinton joining Barack Obama and Michelle Obama — you know it sounds more like one of those reality shows.”

A former Senate aide who has long been promoting the idea of Mrs. Clinton as Senate majority leader, Steven Clemons, said the possibility has been all but foreclosed for now because Senator Reid of Nevada has said he has no plans to step aside. Mr. Clemons called offering the vice presidency to Mrs. Clinton “the politically mature thing to wow a person like myself.”

“There’s nothing John McCain could do to beat an Obama-Clinton ticket,” Mr. Clemons said.

Mr. Obama could try to improve his standing with women and avoid some of the pitfalls of selecting Mrs. Clinton by picking another woman, such as Governor Seblelius of Kansas. However, several observers said such a maneuver might backfire.

“If it looks anything like tokenism, you’re going to anger a lot of the very people you are trying to win over,” Mr. Panetta said. “You’ve got a major leaguer in Mrs. Clinton. She may strike out every now and then, but she hits a lot of home runs. Are you going to bring someone like that on the team or are you going to reach to the bench?”

Still, one lawmaker squarely in Mrs. Clinton’s corner said the idea of Mrs. Clinton joining the ticket was moving from pure fantasy to one with real momentum. “A month or so ago, I wouldn’t have thought it was within the realm of possibility,” Rep. Joseph Crowley, who represents Queens and the Bronx, said. “I think today it really is.”

A top surrogate for Mrs. Clinton, Lanny Davis, told the Sun last night that he was starting a petition to urge her selection for the vice presidential slot.

“I don’t believe we’re playing the politics of regionalism,” another Clinton backer, Rep. Sheila Jackson Lee of Texas, said. “We need to look to the person who documented 18 million votes and another person who documented 17-18 million votes. That’s what makes for a victory.”

Rep. James Clyburn of South Carolina, who was courted aggressively by both candidates before endorsing Mr. Obama yesterday, said he welcomed the talk of an Obama-Clinton ticket. “It would not be insulting to me at all,” Mr. Clyburn told MSNBC.