Update to this story.

DENVER (AP) — President Barack Obama on Wednesday will headline his first fundraiser for a Senate Democrat in danger of losing this fall — but the candidate, Colorado Sen. Mark Udall, won’t be by his side.

In a last minute switch, Udall’s campaign says the senator plans to stay in Washington to vote on Obama’s nominee to lead the Department of Housing and Urban Development. The decision is likely to spark new questions about the political risks for vulnerable Democrats in being linked with an increasingly unpopular president.

“Mark is grateful for the president’s support, and had hoped to welcome him to Colorado in person, but his responsibilities to serve Colorado in the Senate come first,” spokesman Chris Harris said.

Udall had already been planning to limit his appearances with the president. The fundraiser is off limits to news cameras. And Udall’s campaign announced earlier in the week that the senator would not attend the president’s economic speech in Denver Wednesday morning, ensuring that there would be no photos of the two men together.

Udall is one of several Democratic senators at risk of losing their seat this fall, putting Senate control within GOP reach for the first time during Obama’s presidency. Losing control of the Senate would be devastating to Obama’s chances of passing any major legislation in his final two years in office, though his prospects may be little better if Democrats hang on to the Senate and Republicans maintain their majority in the House.

While Obama has called keeping the Senate one of his top priorities, he is limited in how much direct involvement he can have in helping the most at-risk members of his party. Like Udall, most are from swing states or conservative-leaning states like Louisiana, Arkansas and North Carolina, where close ties with the president may be more of a hindrance than a help.