Republican presidential hopeful John McCain and his Democratic rival Barack Obama are in the final stretch of the process of selecting their running mates -- even if they won't openly say so.

Asked about his timeline for announcing a choice, and how many potential number-twos he was considering, McCain sidestepped the question.

"I would love to, but I can't. And I'm sure you understand," he told the ABC network on Monday.

Mark Salter, a close McCain adviser, added: "I've been instructed on the penalty of death that I give no clues as to where the process is."

But some are claiming insight into when the announcement will be made.

"Sources close to Sen. John McCain's presidential campaign are suggesting he will reveal the name of his vice presidential selection this week while Sen. Barack Obama is getting the headlines on his foreign trip," veteran Washington reporter Robert Novak wrote on his blog Tuesday.

"The name of McCain's running mate has not been disclosed, but (former Massachusetts governor) Mitt Romney has led the speculation recently."

The McCain camp offered no credence to the blog, with an aide merely stating Tuesday that there would be "no annoucement today."

But South Carolina Senator Lindsey Graham, a close friend to McCain, fueled the speculation about Romney.

"I think he's very much a contender for the job," Graham was quoted as saying on the website of congressional newspaper The Hill.

It was all quiet on the Obama front.

"We're not speaking to anyone. This is for Senator Obama to share at the appropriate time," said Eric Holder, whom Obama asked to vet potential vice presidents along with Caroline Kennedy, daughter of president John F Kennedy.

But even as they keep the lid on things, both of their parties' presumptive nominees have been offering clues as to who their running mates may be.

Romney, McCain's one-time bitter rival for the Republican White House nomination, has emerged as a possibility for McCain, and the two men campaigned side by side in Michigan on Friday.

"Mitt has been of tremendous help to my campaign.... He does a better job for me than he did for himself," McCain quipped Tuesday at a town hall meeting in New Hampshire when asked if he had mended fences with his former rival.

Picking Romney, a business executive, "given the down economic times that we're living in... would give McCain a layer of credibility on economic issues," said Thomas Whalen, a political historian and professor of social science at Boston University.

Last week at a rally in Indiana, Obama appeared with two people widely named as possible vice presidential timbre: Indiana Senator Evan Bayh, a former supporter of Obama's nomination rival Hillary Clinton, and Sam Nunn, a former Georgia senator with ample national security credentials.

On his recent visit to Afghanistan and Iraq, Obama took the stage with two other possible running mates, Nebraska Senator Chuck Hagel -- a Republican who opposes the Iraq war -- and Rhode Island Senator Jack Reed, both of them defense experts.

And they are not Obama's only "could bes" -- also in the mix are Joe Biden, current head of the Senate foreign relations committee, Virginia Governor Tim Kaine and Kansas Governor Kathleen Sebelius.

On the Republican side Tim Pawlenty and Charlie Crist, governors of Minnesota and Florida respectively, are frequently mentioned.

The idea of an Obama-Clinton ticket seems a somewhat improbable wild card.

Obama was expected to announce his pick ahead of the party convention August 25-28, although analysts say it is unlikely he will do so during the Olympic Games in Beijing August 8-24, which Americans are expected to follow closely.

A few have floated the date August 4, when Obama turns 47.

McCain has more time and could wait until after the Democratic convention. If he announces on August 29, he would deflect media attention to his birthday; he'll be 72. The Republican convention starts September 1.

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