When I interviewed George McGovern last summer about the Democratic presidential field last year, the liberal icon expressed the most enthusiasm about the candidacy of Illinois Senator Barack Obama.

But he said he was still talking to the candidates.

New York Senator Hillary Clinton talked with McGovern a lot, recalling her work in Texas for his 1972 presidential campaign and reassuring the former South Dakota senator that she really was determined to end the war in Iraq.

In October, he flew to Iowa and endorsed her candidacy, while saying nice things about Obama and several of the other contenders.

On Wednesday morning, after watching the candidates spin the results of the North Carolina and Indiana primaries, McGovern came to the conclusion that many savvy political observers have reached: Clinton is not going to be the 2008 Democratic presidential nominee.

“It certainly was not out of any less respect for Senator Clinton. I think she has waged a really courageous and valiant campaign. She will have my affection and admiration for all of my days,” McGovern said. “But I think mathematically the race is all but won by Barack Obama…”

As someone who is familiar with the damage done to frontrunners by extended primary campaigns — he was badly weakened by attacks from the campaign of former Vice President Hubert Humphrey before the California primary in June of 1972 — McGovern took three steps.

First, he announced that he was switching his endorsement to Obama. That made him one of the most prominent Clinton backers to do so. (Georgia Congressman John Lewis, an early Clinton supporter, has made a somewhat similar switch, announcing that he will cast his super-delegate vote at the Democratic National Convention for the candidate who won his congressional district: Obama.)

Second, he called on Clinton to quit the race, saying, “The time has come for all of us to unite and get ready for the general election in the fall.”

Third, he called Bill Clinton, a McGovern staffer from 1972 who appointed the South Dakota as the U.S. ambassador to the United Nations (UN) Food and Agriculture Agencies, and explained his decision.

“I have had many conversations with him over the years, none better than today,” said McGovern. “He did me the honor of talking to me about this. There will be no hard feelings with him or Senator Clinton.”

McGovern’s a class act. He would have made the call no matter how difficult the circumstance. The fact that it turned out to be a “none-better-than” conversation suggests that perhaps even Bill Clinton is beginning to recognize the emerging realities of the 2008 contest.