As Barack Obama's lead over Mitt Romney in the polls narrows, and his presumed fundraising advantage seems about to become a disadvantage, it's alibi time for some of his backers.

His problem, they say, is that some voters don't like him because he's black. Or they don't like his policies because they don't like having a black president.

So, you see, if you don't like Obamacare, it's not because it threatens to take away your health insurance, or to deny coverage for some treatments. It's because you don't like black people.

This sort of thing seems to be getting more frequent, or at least more open. As White House Dossier writer Keith Koffler notes, HBO host Bill Maher accused Internet tyro Matt Drudge of being animated by racism because he highlights anti-Obama stories.

MSNBC's Chris Matthews asked former San Francisco Mayor Willie Brown if House Chairman Darrell Issa's treatment of Attorney General Eric Holder was "ethnic." Brown agreed, and Matthews said some Republicans "talk down to the president and his friends."

There's an obvious problem with the racism alibi. Barack Obama has run for president before, and he won. Voters in 2008 knew he was black. Most of them voted for him. He carried 28 states and won 365 electoral votes.

Nationwide, he won 53 percent of the popular vote. That may not sound like a landslide, but it's a higher percentage than any Democratic nominee except Andrew Jackson, Franklin Roosevelt and Lyndon Johnson.

Democratic national conventions have selected nominees 45 times since 1832. In seven cases, they won more than 53 percent of the vote. In 37 cases, they won less.

That means President Obama won a larger percentage of the vote than Martin Van Buren, James K. Polk, Franklin Pierce, James Buchanan, Grover Cleveland, Woodrow Wilson, Harry Truman, John Kennedy, Jimmy Carter and (though you probably don't want to bring this up in conversation with him) Bill Clinton.

Now it is true that you can go out in America and find people who would just never vote for a black person. But it's a lot harder than it was a generation or two ago, when most voters admitted to pollsters they would never vote for a black president.

And you can probably find some people who usually vote for Democrats but would not vote for a black Democrat. But not very many of them, and they're likely to be pretty advanced in age, and so there are likely fewer of them around than there were four years ago.

My own view is that such voters were more than counterbalanced by voters who felt that, as an abstract proposition in the light of our history, it would be a good thing for Americans to elect a black president.

In 2008, Obama, who came to national attention by decrying the polarization of Red-state and Blue-state America, had obvious appeal to voters. I think there is a similar, and similarly unquantifiable, factor working for Obama this year: Many voters feel, as an abstract proposition, that it would be a bad thing for American voters to reject the first black president.

Some conservatives complain that there is a double standard, that whites who vote against Obama are accused of racial motives, while blacks, 95 percent of whom voted for him, are not.

I think that's unfair. Members of an identifiable group that has been in some way excluded from full recognition as citizens will naturally tend to support a candidate who could be the first president from that group. In 1960, Gallup reported that 78 percent of American Catholics voted for John Kennedy.

American blacks have suffered exclusion and discrimination more than any other group. And very large percentages of them regularly vote for candidates who share Obama's views on issues.

What's remarkable about our politics in 2008 and today is that most voters seem to be making their decisions based on their assessment of the issues and the character of the candidates.

The fact that some have, at least for the moment, moved away from supporting Obama to opposing him, or remain unsure, reflects not an increasing racism, but the fact that we simply have more information than we had four years ago.

Most of us are disappointed when our candidates don't win. But that's no excuse for phony alibis.