The United States under an Obama administration would see "less prosperity, and less security," said Romney, in what could have been an 11th-hour audition to join Sen.'s ticket.
The former Massachusetts governor led a Republican delegation to the convention city to rain on Obama's parade. Obama, though, wasn't even there; he'd decided earlier to methodically wend his way here, making campaign appearances along the way.
Holding a news conference on the fringe of the convention arena, Romney cited Obama votes on taxes and proposals for government spending. And he took note of the Illinois senator's comment earlier this year in debate that he would negotiate with leaders of rogue nations.
Summing up his pitch, Romney questioned the Democrat's judgment, saying: "Barack Obama is a charming and fine person with a lovely family but he's not ready to be president."
McCain's campaign dispatched Romney, who is believed to be a top contender for the vice presidential spot on the ticket, specifically to assail Obama. Another Republican said to be in serious contention for the No. 2 slot, Minnesota Gov. Tim Pawlenty, was scheduled to play attack dog in Denver on Thursday.
The Arizona senator is expected to name his vice presidential pick in the coming days, maybe even while Obama is holding court in Denver. Rallies in Ohio, Pennsylvania and Michigan, perhaps involving the complete GOP ticket, are planned for the run-up to the GOP convention that starts Monday in St. Paul, Minn.
For now at least, McCain's campaign is mum on the deliberations. Uncharacteristically, so, too, is the candidate himself.
And, McCain's prospective candidates are dodging questions about the process to avoid violating McCain's demand for secrecy around his search. But the appearances by Romney and Pawlenty in Denver are certain to stoke speculation and invite questions.
"I don't have anything to tell you about Senator McCain's vice presidential selection process," Romney said repeatedly when asked about his prospects on Tuesday.
Likely by design so top McCain aides could gauge their effectiveness, both Romney and Pawlenty have emerged as top surrogates for McCain over the past few months as he has weighed his choice.
Romney, a former McCain rival, would bring economic credentials and a battle-honed pitch from the rough-and-tumble GOP primary to the team, while Pawlenty, a longtime McCain ally, would bring a solid conservative resume and blue-collar roots.
But both men also present drawbacks for the GOP ticket.
Romney's enormous wealth could exacerbate Democratic attacks on McCain's gaffe over not knowing how many houses he owns. Romney also made unflattering comments against McCain in the primary that the opposition certainly would use against the ticket.
Pawlenty's relative youth, at age 47, could highlight McCain's distinction of potentially being the oldest first-term elected president; he turns 72 on Friday. And Pawlenty isn't as seasoned as others, including Democratic vice presidential pick Joe Biden, in the attack role that a running mate typically plays.
It's possible that McCain could make a nontraditional selection for the GOP ticket, such as abortion-rights backers Tom Ridge, the former Pennsylvania governor, or Democrat-turned-independent Sen. Joe Lieberman of Connecticut. Both are close friends of the candidate.
Either of those options, however, would inflame the GOP's conservative base and risk party-splitting acrimony at the Republican convention next week. Either move also would give Democrats a ready opening to attack the GOP for being divided, much like the story line McCain's campaign is stoking in Denver by openly courting Hillary Rodham Clinton's supporters.
On the other hand, such an out-of-the-box choice could send a message to the public at large - McCain is no George W. Bush, and is no partisan - as the Arizonan seeks to shake the association with the unpopular GOP president and bolster his argument that he would put "country first," ahead of politics.
Given McCain's reputation for doing what he wants, it's also possible that any number of dark horse candidates could emerge. And party officials have been told to prepare for the possibility of an "unconventional" vice presidential nominee.
Even so, Lieberman told reporters after a speech to a business group in New London, Conn., on Monday that he didn't expect to be offered a spot on McCain's ticket, and that he and others close to McCain should take a "vow of silence" concerning the vice presidential nomination.