A commenter named Patience argues that Obama is more likely than Clinton to get a popular-vote mandate:
I think the Democratic candidate will win in 2008, but I also think Barack Obama might have an interesting advantage on Election Day if he were the nominee: I suspect there would be massive turnout in Southern states by African-Americans who didn't vote in 2000 or 2004 because their state's electoral votes were safely Republican but who would be eager to cast votes for the United States' first black president. I don't think this would be enough to tilt the South's Electoral College votes to the Democrats -- except maybe in Florida and Virginia -- but I bet Obama would win the popular vote by a surprisingly large margin. A large margin would do him a world of good in starting his presidency off on a high note.
That's certainly a concievable scenario, but right now the polls show Republican John McCain running at least even with both Clinton and Obama, with McCain closer to getting to 270 electoral votes. Using Alabama and Mississippi to get a bigger popular-vote margin only works for Obama if he can still carry Michigan and Pennsylvania, even if by one vote each, in order to actually win the presidency.
But the question of who can get a bigger popular vote is a reminder that someone may again become president while getting fewer votes than his or her opponent. It happened in 2000, and it nearly happened in 2004. (If Kerry had carried Ohio, he would have won the election, but he still would have finished a couple of million votes behind Bush.) I would love to hear the views of Clinton, McCain, and Obama on the importance of winning the popular vote in order to claim a mandate. If Clinton and Obama say that they would not be troubled by winning in the Electoral Collage and losing the popular vote, they would undermine a lot of Democrats' view that Bush's election in 2000 was illegitimate. And if McCain hints that it would be bad for the country for the Democrats to win by simply flipping Ohio or Florida rather than winning the popular vote -- well, he would essentally be saying that Bush didn't deserve to win in 2000.
After the 2000 election, newly elected Sen. Hillary Clinton came out strongly against the current system of electing presidents, saying, "it's time to do away with the Electoral College and move to the popular election of our president." (See CBS News.) But I'm guessing that none of the three plausible candidates still in the race are going to express any reservations of a repeat of 2000 before the votes are counted in November.