Former President Barack Obama has endorsed Democrat Sara Gideon in her bid to unseat four-term U.S. Sen. Susan Collins in November.

Gideon, Maine’s House speaker, was among five U.S. Senate and 52 U.S. House candidates endorsed Monday by Obama, who also urged people to vote early, either by mail or in person.

I’m proud to endorse this diverse and hopeful collection of thoughtful, empathetic, and highly qualified Democrats. If you’re in one of their districts or states, make sure you vote for them this fall. And if you can, vote early—by mail or in person. https://t.co/PSm3Rf3wkF pic.twitter.com/7RXrJriBz8

— Barack Obama (@BarackObama) August 3, 2020

The race between Gideon and Collins is expected to be one of the most closely watched contests in the country. Democrats are not only trying to defeat President Trump, they also are hoping to regain control of the Senate, where Republicans hold a six-seat majority, and they see Collins as vulnerable. Already, the race has featured unprecedented levels of spending and advertising, and that’s certain to continue into the fall.

There hasn’t been much public polling on the race so far, but a survey of 888 likely voters conducted by Colby College and released last week showed Gideon with a 5-point advantage. Collins won her last race, in 2014, with 68.5 percent support.

The Colby poll, conducted July 18-24 – not long after Gideon officially won the Democratic nomination – showed her with a 44 percent-39 percent advantage over Collins, with 12 percent undecided and 6 percent saying they planned to support another candidate.

I'm honored to receive an endorsement today from @BarackObama. There is so much at stake in this election, and it’s going to take all of us working together to defeat Senator Collins in Maine and flip the Senate.

Join our team: https://t.co/COv597duW3 pic.twitter.com/aThv3Pm81R

— Sara Gideon (@SaraGideon) August 3, 2020

Independent Tiffany Bond and Green Independent Lisa Savage, who also will be on the ballot, were not named as options. The poll also did not ask about ranked-choice voting, which will be used in November.