Philip Bump:

Well, the main obstacles are twofold, I think.

The first is that the Senate — only a third of the Senate is up in any given year. And so that makes it challenging. You can't simply just overhaul the Senate all at once. The second challenge is that a lot of these seats that they could pick up, a lot of the incumbent senators who are up on the ballot, are in pretty red states.

And, obviously, the redder the state, the tougher it's going to be for a Democrat. So we have, if you look at what — Cook Political Report, they do general assessments of the state of the race periodically.

If you look at what they say right now, there are three states where the Democrats could pick up seats. Those are in Colorado, in Maine and in Arizona. Now, none of those is necessarily a gimme, but two of those states at least voted for Hillary Clinton in 2016.

At the same time, though, the Democrats also will need to defend the seat that they won in the special election and Alabama, Doug Jones' seat. And that's going to be a challenge. I mean, Alabama is a very, very red state. And so they have to defend that. They have to win the three states I just mentioned.

And if they do that, they're still one seed short of getting to the 50-50 level in the Senate, which would essentially serve as a majority.