America, meet the new lefty/MSM meme.
Carville is a savvy politico and hard-core partisan. He realizes that Barack Obama can't, at present, hope to win re-election on the strength of his own record, as Ronald Reagan did in 1984. Thus, Obama's best hope is to demonize and marginalize the GOP and its candidate enough that independents decide the President is the lesser of two evils, and hold their nose but vote to re-elect him.
Putting Donald Trump front and center as a Republican candidate suits left-wing/MSM purposes because his poor grasp of public policy and flaky ideas will stigmatize the party in the eyes of independents to the extent that the GOP embraces him.
As Carville well knows, Trump's views are not conservative, and do not reflect those of most Republicans. As Club for Growth has pointed out, Trump isn't even a free-trader, and he's supported tax hikes and universal health care.
What's more, many Republicans continue to mistrust Mitt Romney for his evolving views on some policy issues. Given that suspicion of perceived ideological shifts and the party's deep pro-life commitment, is it really reasonable to expect that they're solidly behind a guy like Trump, who just became pro-life in APRIL of 2011?
Everyone knows these are serious times, and serious times demand serious leaders. James Carville WISHES that Donald Trump would seize the Republican mantle, because he's one of the few people who make President Barack "Lead-From-Behind" Obama look serious.