Update, 9:35 p.m.: Martha Roby has won the runoff in Alabama’s Republican primary race for the Second Congressional District.

Representative Martha Roby of Alabama is battling for political survival in a Republican runoff primary on Tuesday. She faces an aggressive challenger and lingering suspicion among conservative voters about her support for President Trump.

Polls will close at 8 p.m. Eastern on Tuesday night, and we will have live results shortly after. Here’s our guide to the race:

Why was Martha Roby in trouble in the first place?

The short answer is: “Access Hollywood.” Ms. Roby withdrew her endorsement of Mr. Trump in 2016 after the release of a recording that showed him boasting about physically accosting women. His behavior, Ms. Roby said, made him “unacceptable.”

But a few weeks later, Ms. Roby’s solidly conservative district — which includes Montgomery, the state capital, and stretches across Alabama’s rural southeast — voted overwhelmingly for Mr. Trump. His surprise victory over Hillary Clinton put Ms. Roby suddenly at odds with the incoming president and leader of her party.