CHARLOTTE, N.C. — One of the things that struck nearly all observers about Barack Obama’s 2008 presidential campaign was that there were times when he was just plain lucky. There was never any better example than his August 28, 2008 nomination acceptance speech at Denver’s Invesco Field.  Holding a giant outdoor event is always a risk; bad weather could turn it into a fiasco, and even somewhat bad weather could keep some of the crowd away.

But August 28 was a gorgeous night in Denver, and Obama got precisely the event he wanted.  Not that he did anything particularly memorable himself: just try to remember what he said in his uninspiring acceptance speech.  But the setting, the crowd, the Greek columns: memorable. Anyone could be forgiven for thinking that, however mysterious it might seem, Obama had higher powers on his side.

Now, as he prepares to accept his party’s nomination in Charlotte, Obama’s luck has changed. The dominant feeling among independent-minded voters who made up his margin of victory in 2008 is disappointment with the president. The best that can be said of them, from Obama’s point of view, is that they haven’t yet concluded that Mitt Romney is an acceptable alternative.  But they’ve already decided that Obama doesn’t deserve another four years in the White House.

Still, the Obama campaign sometimes can’t come to grips with the fact that it’s not 2008 any more. And so convention organizers again planned for him to accept the nomination in a large, open-air stadium, this time Charlotte’s Bank of America Stadium, filled to the top with between 65,000 and 70,000 enthusiastic supporters.

But Obama’s luck has run out. First there were reports that the campaign was having trouble making sure that there would be a full house. Maybe it was lack of enthusiasm among voters, or maybe it was the logistics of the Charlotte area.  Whatever the reality, there were real fears of a less-than-spectacular turnout.

But then, probably more decisively, there was the weather in Charlotte.  Each day of convention week, there has been a serious downpour in the afternoon. And not just rain; on some occasions the sky turned a blackish-green color that looked as if tornadoes might be just around the corner. Last weekend, torrential rains damaged a much-publicized Obama sand sculpture.  A big rain on Thursday night would wash away any remnants of the idea that Obama is a charmed candidate. The contrast between soggy Charlotte and beautiful Denver would be irresistible for pundits everywhere.

So the Obama campaign took the safe route and cancelled the Bank of America Stadium event, moving it to the Time Warner Cable arena, site of the convention’s first two nights. The arena holds less than one-third of the people the stadium holds, so there’s no question it will be full. But there’s also no question that it will be less than Obama wanted.

In any contest, a competitor has an advantage if his rivals believe he is somehow charmed, somehow ordained to win.  That was Obama in 2008. That’s not the Obama of 2012.