The prospect of an Obama-Clinton ticket has been fodder for political gossip for months, with some Democratic leaders pushing the idea as a way to unify the party.

The Obama and Clinton campaigns have consistently shrugged off the idea, however, and Mrs Clinton has been adamant that she is interested only in the presidency.

Yet anyone who knows the Clintons is well aware that, at times, they come to politics with different motivations. Both of them want to return to the White House; Mrs Clinton, representing New York, also enjoys being a senator, while Mr Clinton, according to associates, sees the vice-presidency as perhaps her best path to becoming president some day if she loses the nominating fight.

And he has his own ideas about his wife's best interests - even if she sometimes does not share them.

A spokesman for Mrs Clinton's campaign said on Thursday that Mr Clinton has not had private conversations in which he is pushing her for the vice-presidency or arguing that she deserves it, and that he believes the choice of a running mate is a personal one for the nominee.

Friends of the former president say his musings have been more casual: He believes an Obama-Clinton ticket could help unify the party, and he thinks she has earned a meeting with Mr Obama to discuss that possibility.

According to these friends, Mr Clinton believes his wife's victories in major primary battles, such as in Ohio and Pennsylvania, and the 16 million votes cast for her candidacy make her the proper choice for Mr Obama's running mate.

'If she's not going to be the nominee, then he wants her in the second spot,' said one friend of the Clintons.

'In the long run, it's the best way for her to run again in 2016.'

Time magazine first reported Mr Clinton's interest in the No. 2 slot for his wife on Thursday.

But Clinton advisers were emphatic that neither Mr Clinton nor anyone else in the campaign had given up on Mrs Clinton's candidacy, and they emphasised that no efforts were being made to position her as running mate to Mr Obama.

'Senator Clinton is solely focused on being the Democratic nominee,' said Mr Howard Wolfson, the Clinton campaign's communications director.

Meanwhile, Democratic officials say Mr Obama has begun a top-secret search for a running mate, with the goal of having an early list of names to begin sifting through shortly after the final two primaries on June 3.

The party's likely presidential nominee has asked Mr Jim Johnson, former CEO of government-sponsored mortgage giant Fannie Mae, to begin vetting potential vice-presidential picks.

Mr Johnson had the same job for Democratic nominees John Kerry in 2004 and Walter Mondale in 1984.

Mr Obama's campaign has refused to talk about who was being considered, but possible options are Mrs Clinton; governors such as Arizona's Janet Napolitano, Ms Kathleen Sebelius of Kansas and Mr Tim Kaine of Virginia; foreign policy experts such as former Georgia senator Sam Nunn, Connecticut Senator Chris Dodd or Delaware Senator Joe Biden; or other senators such as Missouri's Claire McCaskill and Virginia's Jim Webb.

Mr Obama could also look outside the party to people like war critic Nebraska Republican Senator Chuck Hagel or independent New York Mayor Mike Bloomberg. Or he could even look to one of his early prominent supporters like former senator Tom Daschle of South Dakota or try to bring on a Clinton supporter like Indiana's Evan Bayh.

NEW YORK TIMES, ASSOCIATED PRESS