U.S. Sen. Cory Gardner of Colorado is scheduled to attend a “Save the Senate” retreat next month with President Donald Trump and several other Republican senators at the Trump International Hotel in Washington, D.C.

Trump and Gardner, who have endorsed each other’s 2020 re-election bids, are among the “special guests” listed on an invitation to the two-day event, which is sponsored by the National Republican Senatorial Committee and was first reported Friday by online news site The Intercept.

The other senators on the invitation are NRSC Chairman Todd Young of Indiana, Senate Majority Leader Mitch McConnell of Kentucky, Shelley Moore Capito of West Virginia, Steve Daines of Montana, Jim Risch of Idaho, Pat Roberts of Kansas, Dan Sullivan of Alaska and Thom Tillis of North Carolina.

Michigan Senate candidate John James is also listed among the special guests.

Gardner is considered the most vulnerable Republican senator on the ballot ahead of next year’s election, when Democrats will have to win three or four seats – depending which party controls the White House – to win a majority in the Senate.

Scheduled for Nov. 7 and 8, the event falls just more than a month after Gardner joined former U.N. ambassador Nikki Haley and Donald Trump Jr. to headline a three-day fundraiser in New York for Trump’s 2020 campaign, the Republican National Committee and several state GOP committees.

Spokespeople for Gardner’s campaign and the NRSC didn’t respond to a request for comment.

Gardner and Trump are both underwater with Colorado voters, according to polls released this week.

The Democratic Keating-OnSight-Martin survey released Thursday found Gardner is viewed unfavorably by 45% of Colorado voters and favorably by just 34%, while Trump is viewed unfavorably by 60% of the state’s voters and favorably by 38%.

The Morning Consult’s latest quarterly Senator Approval Rankings, also released Thursday, found Gardner is viewed favorably by 36% of Colorado voters and unfavorably by 39%, landing him among the 10 least popular senators in the country according to the firm’s analysis. Trump gets thumbs-down from 56% of Colorado voters and approval from 41%.

Nine Democrats are running in a primary for the chance to challenge Gardner. The Keating-OnSight-Martin found the frontrunner, former Colorado Gov. John Hickenlooper, leading Gardner 53% to 42%.