Lord knows Obama shouldn’t be overconfident. I’ve argued before and will argue again that complacency is Obama’s enemy. The simple fact is that in 2008 — in the worst possible year for Republicans and after being vastly outspent by Obama — John McCain still managed to get 46 percent of the vote. Yes, for all his troubles, Romney starts with a real base.

Nonetheless, we have decidedly passed from a time when Obama looked very vulnerable to a moment when he now has to be declared the favorite. What prompts this observation is a very enlightening story in today’s Post by Jon Cohen, the paper’s director of polling. The basic finding from Washington Post-ABC News polling: Romney’s negatives have soared while Obama is, after all the fights of the past three years, still viewed favorably by a majority of Americans.

Cohen reports that while Obama is viewed favorably by 53 percent of Americans, Romney is viewed favorably by only 34 percent. Even more significantly, Obama is viewed unfavorably by only 43 percent of Americans, compared with 50 percent who view Romney unfavorably. Thus, Obama’s net favorable is plus-10, Romney’s is minus-16.

Cohen’s bottom line about Romney: “As he has become better known, his unfavorables shot up far more rapidly than his positive numbers. Negative impressions are up eight percentage points in the past week, nudging past the precious high….”

This may help explain Post blogger Greg Sargent’s report on new Quinnipiac polling that found Obama leading in three key swing states — by 49-42 in Florida; 47-41 in Ohio; and 45-42 (which, as Greg points out, is statistically insignificant) in Pennsylvania. Without Ohio and Florida, Romney can’t win. It should be said, though, that Obama’s statistically insignificant lead in Pennsylvania should worry his campaign. Pennsylvania has been drifting Republican, and if Romney won it, he would be stealing a state that has, over the past several elections, been part of the Democratic base.

The last shows why eternal vigilance is the Obama team’s job. But the other numbers suggest that Romney has become a far weaker candidate than many in both parties thought he would be when this campaign started. (I am among those surprised that Romney has not been a stronger campaigner.)

The findings that Cohen highlights also suggest something else: That to win key states in the primaries, including Ohio and Florida, Romney’s side — mostly through his super PAC — spent a fortune attacking his opponents, but did little to build the candidate himself up. This contrasts sharply with the 2008 race between Obama and Hillary Clinton, when voters in key states saw a good deal of positive campaigning. And shrewd conservatives I have been talking with are worried that Romney is weak in a constituency that has been vital for Republicans in recent elections: white working class voters.

Again, it’s a long way to November, and the economy and perceptions of it will remain vital to the outcome all the way through. But Cohen’s article is an important straw in the wind. Obama has become the favorite. He just can’t let it go to his head.