The competitive map for control of the U.S. Senate has expanded considerably in the final days before Election Day. Here's a ranking of those competitive races from most likely to flip to least likely.

(Image credit: Megan Varner/Getty Images)

The competitive map for control of the U.S. Senate has expanded considerably in the final days before Election Day. Here's a ranking of those competitive races from most likely to flip to least likely.

(Image credit: Megan Varner/Getty Images)

Democrats hold significant advantages going into Election Day, according to a new NPR/PBS News/Marist poll. But lagging enthusiasm among key groups is just one indicator that gains may not come easy.

(Image credit: Jason Miller)

Democrats hold significant advantages going into Election Day, according to a new NPR/PBS News/Marist poll. But lagging enthusiasm among key groups is just one indicator that gains may not come easy.

(Image credit: Jason Miller)

With Election Day just a few weeks away — and millions of votes already cast — midterm polls are everywhere. While polling can be a helpful tool to understand the electorate, it's not a crystal ball.

(Image credit: Tom Brenner)