He will lose, Much worse than in 1972. McGovern is too unpopular to the moderate Americans.
I doubt he would have done much worse than Mondale (the 41% of the electorate that voted for Mondale would have probably voted for just about any Democratic nominee; they were the diehards in a Republican landslide), although just worse enough that Reagan would have managed a 50-state sweep, with an Electoral College count of 535-3. Mondale barely carried his home state of Minnesota, which to this day hasn't voted Republican since (you guessed it) 1972. Mondale pretty much got the same people McGovern got 12 years earlier, only they were a slightly larger portion of the population by that time.
While I know this isn't the conventional view, in my estimation, the country has been moving leftward since 1960 (in the partisan sense of being more friendly to Democrats, not necessarily in absolute, ideological terms), a fact which was occassionally obscured by shorter term trends (most notably, Reagan's intense personal popularity during the 1980s, and Clinton's disastrous first two years in office handing the GOP control of both Houses of Congress in '94). The fact that even a Michael Dukakis, who lost 40 states, was still able to capture Oregon (which hadn't voted Democrat since 1964, Washington state (which hadn't voted Democrat since 1968), and came very close in states like Missouri and, of all places, Montana, is indicative of the early fruition of a long-term shift away from Republicans, and towards the Democrats, that would only accelerate after the end of the Cold War. What I suspect will be Obama's minor landslide victory later this year will be the ultimate fruition of that shift (quite possibly fortelling a long-term trend in the other direction, although that may not begin until the 2020s or whatnot).
Anyway, not trying to take this thread off-topic with a lot of partisan chitchat, but sometimes its difficult to discuss alternative history without going into the past and near-future. And anyway, my political views are obscure enough that I can assure one and all that I have said nothing intended to advance them in this post.