Less than two months ago, Michael R. Bloomberg told a PBS interviewer that he probably couldn’t win a Democratic primary, and that entering the race as late as November would be “pretty hard.”

Well, it’s November now, and he seems to have changed his mind. Here’s a look at what happened on the campaign trail this week.

And then there were 18?

Mr. Bloomberg, the former mayor of New York, filed paperwork on Friday to put his name on the ballot for the Democratic primary in Alabama.

According to his advisers, he has not definitively decided to run for president, but the paperwork was necessary to keep the possibility alive, because Friday was Alabama’s filing deadline. Arkansas and New Hampshire have deadlines next week, and Team Bloomberg says it plans to meet those, too.