WASHINGTON A possible match-up in the November presidential election would pit a crusty old politician known for blowing his top against a first-term lawmaker seen as charismatic yet largely untested.
In that scenario Republican John McCain of Arizona would face Democrat Barack Obama of Illinois.
Or McCain would go against the tenacious, politically savvy, two-term Democratic Sen. Hillary Clinton of New York.
Whoever wins the Democratic race -- McCain sewed up the Republican nomination on Tuesday -- it will be the first time two sitting members of the Senate ever face each other in the general election for the presidency.
That adds to the possible firsts in this White House race -- McCain, who turns 72 in August, would be the oldest person ever to be elected for a first term as U.S. president, Obama would be the first black president and Clinton would be the first woman president. One of them would also be the first sitting senator elected president since John F. Kennedy in 1960.
Inside the 100-member U.S. Senate, a clubby institution often referred to as "the world's most deliberative body," all three have reputations for reaching out to members of both parties -- though McCain far more forcefully.
Obama, 46, who is leading 60-year-old Clinton in a drawn-out race for the Democratic nomination, would provide the starkest contrast with McCain.
Dubbed "Senator Hothead," McCain has challenged, bullied or even cursed lawmakers over a host of issues the past two decades -- from cutting wasteful spending to reducing the influence of money in politics to revamping U.S. immigration laws.
"A lot of McCain's colleagues just don't like him, Democrats and Republicans," said Norm Ornstein, a congressional scholar at the American Enterprise Institute.
"I think there are two reasons. One, they felt the wrath of his temper ... or two, he's challenged them on matters they didn't want to be challenged on," Ornstein said.
Obama has generally deferred to his elders on Capitol Hill since he arrived in the Senate in 2005 as a rising political star -- one seen as liberal, amiable, and a powerful speaker.
"He kept his head down, worked hard, didn't try to use his star power to big-foot any issue and went out of his way not to tick anybody off," said a Democratic leadership aide.
Obama soon drew McCain's ire. They clashed in 2006 over a bill to tighten controls on lobbying Congress. The Arizona Republican accused the Illinois Democrat of "disingenuousness." Obama denied it and suggested McCain had overreacted.
More recently, McCain, a Vietnam war hero, accused Obama of being naive in opposing the Iraq war. Obama fired back by charging that McCain had recklessly helped lead the United States into the Iraq war.
SHIFT IN CONGRESS
In Congress, attitudes toward the presidential hopefuls in their midst have shifted as the campaigns progress.
Congressional Democrats had initially favored Clinton of New York, wife of former President Bill Clinton, for their party's 2008 presidential nomination.
As the first former first lady ever elected to the Senate, she took office in 2001 and quickly won praise on both sides of the political aisle as an effective and hard worker. Outside the Senate, she remained a popular yet polarizing figure.
Many Democrats in Congress backed Obama for president after the former Chicago community organizer began to win state-nominating contests earlier this year while preaching "hope and change" in Washington, long paralyzed by partisan bickering.
Assistant Senate Majority Leader Dick Durbin was the first Democrat in Congress to back Obama. He did so after helping persuade the freshman senator to run for president.
"I saw in Barack special qualities. His speaking ability, his energy, his optimism, his ability to connect with people," said Durbin, who like Obama, is from Illinois.
"I told him, 'You can't pick the time. The time picks you. Don't think sitting around here casting another 1,000 votes will make you a better candidate for president,'" Durbin said.
Rep. Ray LaHood, an Illinois Republican, has endorsed McCain for president, but speaks well of Obama, too.
"Two days after he (Obama) was elected to the Senate, he called me and said, 'I want to sit down and talk with you about how we can work together for your district and our state,'" LaHood recalled, saying they joined forces on legislation.
"He believes in trying to bring people together. It's not a phony campaign slogan. This is genuine," LaHood said.
As for McCain, LaHood said: "People respect him. He's an independent-minded guy. Maybe he has blown his stack once in awhile. But John's gotten things done."
Many congressional Republicans, though, were wary about McCain's White House bid because of his temper and willingness to cut deals with moderate and even liberal Democrats.
But as he emerged as the frontrunner, they rallied behind him, including one he swore at last year.
Republican Sen. John Cornyn of Texas was the object of a McCain tirade at a meeting about a measure that McCain had helped draft to revamp U.S. immigration laws.
"He apologized shortly afterward," Cornyn said. "John is passionate. I don't begrudge him his passion. He's a maverick. That's part of his charm."
(Editing by David Alexander and Frances Kerry)
(For more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at blogs.reuters.com/trail08/)