For whatever reason, people in Minnesota really seem happy with their decision to keep Al Franken in Congress. Despite his nebulous messages, boring demeanor, and uniformly terrible speeches on the Senate floor, he trounced his GOP rival in 2014 and remains bafflingly popular. Couple that with the whole "famous comedian" thing and you had what many on the left thought was a recipe for a presidential run.
We've been hearing rumors almost since Franken claimed his Senate seat - at least as far back as 2010 - mostly due to the fact that Dems have an incredibly shallow bench. You may recall that back in February, the National Journal was hyping Franken as the left's possible 2020 savior.
However, Franken has now made it clear that it's not going to happen. He'll have to hang on to his one man mobile uplink unit, because he seems to realize he's not good enough, not smart enough, and (Minnesota aside) people don't like him.