Perhaps the most remarkable story of McCain's temper involved Arizona Congressman Rick Renzi. Two former reporters covering McCain, one who witnessed the following events and one who confirmed the facts provided by the first, relayed it to me as follows: In 2006, the Arizona Republican congressional delegation had a strategy meeting. McCain repeatedly addressed two new members, congressmen Trent Franks and Rick Renzi, as 'boy.' Finally, Renzi, a former college linebacker, rose from his chair and said to McCain, "You call me that one more time and I'll kick your old ass." McCain lunged at Renzi, punches were thrown, and the two had to be physically separated. After they went to their separate offices, McCain called Renzi and demanded an apology. Renzi refused. Apparently this posture made McCain admire him, as they became fast friends.

Of course, McCain's camp denies these types of allegations.

Sen. John McCain's defenders are blasting a new biography that claims the White House hopeful once hurled the most vile of epithets at his wife, Cindy.

Democratic strategist Cliff Schecter claims in "The Real McCain" that the presumptive GOP nominee uttered the shocking slur in front of aides and reporters while campaigning for the senate in 1992.

"At one point, Cindy playfully twirled McCain's hair and said, 'You're getting a little thin up there,'" Schecter writes. "McCain's face reddened, and he responded, 'At least I don't plaster on the makeup like a trollop, you [cunt].'"

McCain spokeswoman Jill Hazelbaker brands the book "trash journalism" and tells us, "The story is completely fabricated."

But some of his flare-ups are well-documented.

During a meeting Thursday on immigration legislation, McCain and Sen. John Cornyn (R-Texas) got into a shouting match when Cornyn started voicing concerns about the number of judicial appeals that illegal immigrants could receive, according to multiple sources -- both Democrats and Republicans -- who heard firsthand accounts of the exchange from lawmakers who were in the room.

At a bipartisan gathering in an ornate meeting room just off the Senate floor, McCain complained that Cornyn was raising petty objections to a compromise plan being worked out between Senate Republicans and Democrats and the White House. He used a curse word associated with chickens and accused Cornyn of raising the issue just to torpedo a deal.

Things got really heated when Cornyn accused McCain of being too busy campaigning for president to take part in the negotiations, which have gone on for months behind closed doors. "Wait a second here," Cornyn said to McCain. "I've been sitting in here for all of these negotiations and you just parachute in here on the last day. You're out of line."

McCain, a former Navy pilot, then used language more accustomed to sailors (not to mention the current vice president, who made news a few years back after a verbal encounter with Sen. Patrick Leahy of Vermont).

"[Expletive] you! I know more about this than anyone else in the room," shouted McCain at Cornyn. McCain helped craft a bill in 2006 that passed the Senate but couldn't be compromised with a House bill that was much tougher on illegal immigrants.

Cornyn's office declined to comment on the incident.

His Senate colleagues have certainly noticed his erratic behavior.

"There would be a lot of people who would have to recalibrate their attitudes toward John," said Sen. Robert F. Bennett (R-Utah), a supporter of Mitt Romney's who has clashed with McCain...

"The thought of his being president sends a cold chill down my spine," Sen. Thad Cochran (R-Miss.), also a senior member of the Appropriations panel, told the Boston Globe recently. "He is erratic. He is hotheaded. He loses his temper and he worries me."

"John was very rough in the sandbox," said former senator Rick Santorum (R-Pa.), who is outspoken in his opposition to McCain's candidacy. "Everybody has a McCain story. If you work in the Senate for a while, you have a McCain story. . . . He hasn't built up a lot of goodwill."

I think McCain's temperament is going to become his single biggest obstacle to winning the election. You can't go around calling people names and throwing punches at people and think it won't come out in the glare of a presidential campaign.