BILLINGS, Mont. (AP) — Montana's incumbent Sen. Jon Tester faces Republican challenger Tim Sheehy in a debate tonight as the three-term Democratic lawmaker fights to hold on to his seat. The outcome of the race could be crucial to control of the Senate, where Democrats hold a slim majority. It's set to become the most expensive political contest in Montana history.

The debate will take place at 7 p.m. on the University of Montana campus in Missoula and will last one hour. The format will allow Tester and Sheehy to present in-depth answers, according to Aaron Pruitt, director of Montana PBS. There will also be an opportunity for direct exchanges and, potentially, for the candidates to question each other. The debate will be simulcast live on Montana PBSMontana Public Radio, Yellowstone Public Radio, online at montanapbs.org/live and on the Montana PBS Facebook page.