BOSTON Republican presidential candidate John McCain once likened tangling with Mitt Romney to "a wrestling match with a pig," saying "you both get dirty, and the pig likes it."
That was back in January when the two were slugging it out in a fierce presidential nomination race, a contest where there appeared to be deep personal hostility between the two Republicans.
Fast-forward six months and Romney's stock is rising as a top choice to be McCain's vice presidential running mate, raising questions of whether the two can overcome personal enmity and join hands to win November's U.S. presidential election.
Political experts say the weakening U.S. economy, the No. 1 issue for voters, could sway McCain to pick Romney, whose experience as a successful former venture capitalist could compensate for McCain's weaknesses in economic matters.
They add that Romney, a multimillionaire former Massachusetts governor, would bring considerable strengths to the Arizona senator's bid for the White House -- from a proven ability to raise big money and connect with conservative Republicans to his telegenic good looks and executive style.
The polished communicator with combed-back salt-and-pepper hair also would add a dose of youthfulness.
Romney is 61 years old, a decade younger than McCain who would be the oldest person ever elected to a first presidential term. Sen. Barack Obama, McCain's Democratic rival for the White House, is 46.
"They are two very different kinds of people. There is clearly a lot of tension between the two," said Julian Zelizer, a history and public affairs professor at Princeton University.
"But that never stops anyone from joining into an alliance if they can win. And given the odds Republicans face and given the challenges that McCain faces in winning, if Romney brings him that one asset that changes his odds, I think McCain would be more than willing to enter into that alliance," he said.
On Monday evening, Romney, who had often dismissed McCain as a "liberal," opened his former rival's campaign office in Michigan, a key battleground in 2008. Romney is an asset there. The son of popular former Gov. George Romney, he won Michigan's January 15 nominating contest decisively.
For his part, McCain told a fund-raising event in Albuquerque, New Mexico, on Monday: "Mitt and Ann Romney and Cindy and I have become good friends," as he described how he feels the Republican Party is united now for the battle against Obama.
There is also historical precedent for two rivals to overcome public displays of animosity to join together in a U.S. general election after a tough nomination fight.
After Lyndon Johnson lost to John F. Kennedy in the 1960 Democratic presidential nomination race, Johnson went on to share the ticket as Kennedy's vice presidential running-mate in a campaign that defeated Republican Richard Nixon.
"Many times open rivals who didn't like each other at all have been on tickets," said Bruce Schulman, professor of political history at Boston University and author of "Rightward Bound: Making America Conservative in the 1970s."
But Schulman remains skeptical of a McCain-Romney ticket, noting that vice presidents have become more powerful in recent years and more reflective of the character of their president. "A close working relationship seems to be important," he said.
He cites President George W. Bush's decision to run with "someone very much like himself" in picking Vice President Dick Cheney, and in former President Bill Clinton's decision to select "another Southern moderate new Democrat from a neighboring state" in running with Al Gore in 1992.
Romney also faces other challenges in joining McCain, who is expected to make up his mind some time before the Republican nominating convention in early September and has said he was not necessarily looking for someone who could help him win a state or a region.
While Romney has cast himself as a Washington outsider who would use his 25 years of business experience to pull the U.S. economy from the brink of recession, some critics question the economic effectiveness of his 2003-2007 term as governor.
While closing a budget deficit, he generated more than $500 million by raising fees and closing corporate tax loopholes -- actions considered tax increases by some businesses.
Opponents also have attacked his credibility, pointing to his shifting positions on issues ranging from abortion to gay rights and gun control. And despite his success in courting conservatives, many powerful evangelical Christian voters still regard his Mormon faith with suspicion.
(Reporting by Jason Szep, editing by Jackie Frank)