MCCLELLAN LATEST REPUBLICAN TO BACK OBAMA…. I vaguely recall the point in the presidential campaign — I think it was called “the summer” — when the chattering class obsessed over whether Barack Obama could keep the various elements of the Democratic Party together.

I think the media missed the more interesting trend: Obama is keeping Democrats together, and is picking up support from conservatives and Republicans.

It’s been a rough couple of weeks for the McCain campaign, watching fairly high-profile Republicans endorse Obama, including Colin Powell, Christopher Buckley, Ken Adelman, and former Minnesota Gov. Arne Carlson.

Yesterday, Obama picked up yet another.

Scott McClellan, the former White House press secretary who sharply criticized President Bush in his memoir last spring, told CNN Thursday he’s voting for Barack Obama.

“From the very beginning I have said I am going to support the candidate that has the best chance for changing the way Washington works and getting things done and I will be voting for Barack Obama and clapping,” McClellan told new CNN Host D.L. Hughley.

McClellan is the third high-profile Bush insider to throw his support to Obama, following Powell and Matt Dowd.

It’s not just the more visible public figure, either. E.J. Dionne, Jr., noted in his column today, “In The Post tracking poll released yesterday, Barack Obama drew 22 percent of the vote from self-described conservatives. That’s a seven-point gain on John Kerry’s 2004 conservative share.”

But it’s the endorsements that have to be dispiriting for the McCain campaign. To be fair, McClellan, by virtue of his book, stopped reading from the Republican script quite a while ago, and it’s fair to say his influence in GOP circles is minimal. But when the spokesperson for a conservative Republican president breaks ranks and throws his support to a progressive Democrat, it only adds to the sense of momentum and inevitability.

Update: Former Massachusetts Gov. William Weld, a Republican, also endorsed Obama this morning.

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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.