If he moves the speech indoors, he's accused of not having enough support to fill the larger venue. If he doesn't move it indoors and it does in fact rain, he's accused of not caring about people's comfort or safety.
40% chance of rain sounds like good enough odds to play it safe and move to the indoor venue.
was the word Tuesday morning. According to campaign manager Jim Messina, Tuesday AM was the cut off point for switching to an indoor venue. The forecast then was 40% rain and today, it's still 40%.
But things have changed and Obama's acceptance speech has now been moved to a much smaller 20,000 seat indoor area because of the chance of rain. I suppose on one hand the move goes right along with Carneys statement a few weeks back that they are reducing crowd sizes intentionally, so that Obama can better connect with the people. However, it could very well be that the DNC is now certain that Obama cannot fill a 70,000 seat stadium anymore and they want to avoid the embarrassment of a bunch of empty seats.