Sen. Mark UdallMark Emery UdallSenate GOP rejects Trump’s call to go big on gun legislation Democratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups MORE's (D-Colo.) last-minute cancellation of an appearance at a high-dollar fundraiser headlined by President Obama may have raised eyebrows, but the president did little to downplay his connection with the vulnerable lawmaker.

Obama heralded Udall as "a serious person who's trying to do the right thing" in a speech to donors and Democratic Party officials gathered at the Denver Westin.

The president said "every single day" Udall was fighting, along with the White House, to provide ladders of opportunities to those who are willing to work hard. He praised the Colorado lawmaker as not an ideologue — and noted they don't agree on everything — but said the pair shared core beliefs on what Democrats should be focused on.

"If you work hard, you should be able to make it,” he said.

Obama implored donors to “feel the same urgency about this race — about Mark’s race, about all the Senate races" as they did for his presidential run "back in 2008."

Proceeds from the fundraiser jointly benefitted Udall's campaign and the Democratic Senatorial Campaign Committee. It was the first fundraiser benefiting a specific candidate that Obama has held this campaign season.

But Udall drew fire from Republicans by opting to remain in Washington instead of appearing alongside Obama.

"Coloradans know that Udall has stood by Obama's side time and time again voting in lockstep 99 percent of the time," said National Republican Senatorial Committee press secretary Brook Hougesen."Mark Udall's disappearing act reinforces why Coloradans have grown tired of his broken promises and are embracing an honest, competent and energetic leader like Cory GardnerCory Scott GardnerTrump makes Manchin top target for midterms Wyden: I object to Trump’s DHS cyber nomination over demands for Stingray information Trump’s confirmation chaos in perspective MORE who solves problems instead of hiding from them."

Gardner, the Republican congressman challenging Udall, released a web video this week knocking Udall's ties to Obama.

But Udall's wife, Maggie, did appear at the event. And a spokesman for the Democrat's campaign defended the decision for the lawmaker to cancel on the fundraiser, saying Udall had remained in D.C. vote to confirm Julian Castro as Secretary of the Department of Housing and Urban Development — a department that helps administer federal flood recovery funds. 

"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," said Udall spokesman Chris Harris.