Mindful of that, Romney will put experience at the top of his list of qualities as he chooses a No. 2, according to senior advisers and GOP operatives familiar with his thinking. They spoke on condition of anonymity because they were not authorized to discuss a process Romney himself is trying to keep as private as possible as he works to narrow a field that may begin with as many as a dozen.
"The hallmark for Governor Romney's candidacy, and how he would be as president, is that he approaches these decisions in a well-thought-out methodical way," said Steve Duprey, a former McCain adviser and current New Hampshire-based member of the Republican National Committee. "It won't be like the McCain campaign where there was a big surprise and effort to create a game changer."
The former Massachusetts governor did give a few hints about his plans Monday, disclosing that he had chosen his former chief of staff and 2008 presidential campaign manager, Beth Myers, to lead the vetting and analysis of prospective running mates.
Romney said the selection would precede the GOP's August convention. Asked about potential choices - and Florida Sen. Marco Rubio, specifically - Romney hedged, as he has before.
Rubio is "one of the terrific leaders in our party, but I think it's way too early to begin narrowing down who the potential vice presidential nominees might be," Romney said in an interview with Diane Sawyer of ABC News outside Fenway Park in Boston. "But we're beginning that process, we'll talk about a lot of folks, and then go through he kind of vetting and review process that you have to go through to make sure whoever you select will pass the evaluation that's required by the American people."
Besides readiness to assume the presidency, Romney has laid out only one other public criterion: that his running mate oppose abortion rights. That could help reassure social conservatives about Romney's opposition to abortion - a sore point because he supported abortion rights when he ran for the Senate in 1994.
Several Republicans familiar with Romney's thinking downplay the importance of choosing a running mate from a battleground state or an important demographic. He also is expected to avoid a candidate whose star power might distract from the campaign's main themes - Republicans are working to make the election a referendum on President Obama - or overshadow Romney.
Rubio, 40, is one such celebrity candidate. And the junior Florida senator also has little experience, in only his second year on Capitol Hill. Still, Rubio is both a conservative favorite and potential bridge to the growing Hispanic voting bloc, which typically favors Democrats.
Ohio Sen. Rob Portman, also high on speculative lists, endorsed Romney early and campaigned for him in the Ohio primary. Romney, who narrowly won Ohio, knows Portman and is said to respect him. The senator has been twice vetted for Cabinet posts, as President George H.W. Bush's trade representative and then director of the Office of Management and Budget.
Romney also is likely to consider conservative favorites talked about often, such as New Jersey Gov. Christie, South Carolina Gov. Nikki Haley and Virginia Gov. Bob McDonnell. Still, if he's looking for experience, that group has just five years' gubernatorial experience among them.
More experienced Republicans might help Romney mitigate some political liabilities. Chief among those vulnerabilities is his wealth and his struggle to connect with working-class voters.
Former two-term Minnesota Gov. Tim Pawlenty's working-class background could help. He has aggressively campaigned for Romney since ending his own presidential run. The GOP budget guru, U.S. Rep. Paul Ryan of Wisconsin, also came from humble roots and campaigned for Romney in Wisconsin's primary - a victory that helped push Rick Santorum out of the race. It's unclear if Ryan's role as face of the Congressional Republican budget plan, which includes a fundamental transformation of Medicare, would present too much political risk.
One certainty: memories of 2008 will be looming large.
"There's one thing the people in the Republican establishment agree on: There was clearly not a thorough thought process or vetting that went into the vetting of Sarah Palin," said Sara Fagen, a former political aide to President George W. Bush. "I don't think they're going to make the same mistake."