A SMALL AND PETTY MAN…. The Senate, as an institution, operates through geniality and camaraderie. Every member identifies every other member as his or her “friend.” When referencing a colleague, it’s a breach of protocol if a senator fails to use the words “gentleman” or “gentlewoman.”

So, last night, when Barack Obama saw John McCain, Joe Lieberman, and Florida’s Mel Martinez chatting on the Senate floor, he did what gracious senators do — Obama crossed the aisle to say hello. Martinez and Lieberman offered warm greetings to their Democratic colleague. McCain? Not so much.

Let the record reflect that Barack Obama made the approach to John McCain tonight.

As the two shared the Senate floor tonight for the first time since they won their party nominations, Obama stood chatting with Democrats on his side of the aisle, and McCain stood on the Republican side of the aisle.

So Obama crossed over into enemy territory.

He walked over to where McCain was chatting with Republican Sen. Mel Martinez of Florida and Independent Sen. Joseph I. Lieberman of Connecticut. And he stretched out his arm and offered his hand to McCain.

McCain shook it, but with a “go away” look that no one could miss. He tried his best not to even look at Obama.

A lot of reporters couldn’t help but notice. Jeff Zeleny and Michael Cooper noted that Obama approached McCain, but McCain “barely pivoted his body as he took Mr. Obama’s hand for a handshake that lasted just a moment.” Carl Hulse reported that McCain only “offered a chilly look” to Obama.

Two months ago, Newsweek’s Jonathan Alter described McCain as a “surprisingly immature politician.” That’s a fair description, but it’s becoming increasingly clear that McCain is also a small and petty man. Worse, as the pressure grows more intense, Obama looks cool, relaxed, and in control, while McCain appears to be cracking.

During last week’s debate, McCain refused to even look at Obama. On Tuesday, asked probing questions by the editors of the Des Moines Register, McCain became sarcastic and visibly agitated. Last night, he couldn’t be bothered to treat his rival with modicum of respect, even after Obama graciously made the first move.

McCain’s emotions are getting the better of him, and revealing a pungent personality. Concerns about McCain’s fragile temperament are obviously justified.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.