By Steve Holland
DENVER (Reuters) - What if President Barack Obama came to your fund-raising event and you did not attend?
That's what happened on Wednesday in Denver when Obama raised money for Colorado Senator Mark Udall, a Democrat who is facing a stiff challenge to re-election in November.
His non-attendance raised questions as to whether he did not want to appear personally with Obama as he tries to fight off a tough challenge from Republican U.S. Representative Cory Gardner. Udall's seat could be key in the Republican drive to capture control of the U.S. Senate.
Udall's campaign said the senator was not able to be at the event because he needed to be in Washington to vote for the Senate confirmation of Julian Castro as housing secretary.
Castro sailed through on a bipartisan 71-26 vote.
"Due to last-minute votes and legislative activity, Mark will be unable to make the trip back to Colorado on Wednesday," Udall's campaign said. "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."
Obama, whose 2008 Democratic presidential nominating convention was in Denver and who won Colorado in 2008 and 2012, made no mention of the fact that Udall was not there when he spoke to the fund-raising crowd.
He urged Colorado voters to have the same urgency for Udall's race as they felt about his race in 2008.
"Mark Udall is a serious person who is trying to do the right thing. ... He's not an ideologue. He doesn't agree with me on everything. But he believes in the core idea that I think should be what Democrats are all about, this idea that if you work hard you should be able to make it."
Obama also acknowledged a difficult environment this election year.
"Every race across the country is going to be challenging, including this one," said Obama, blaming Republicans for blocking much of his agenda.
Obama was introduced at the campaign luncheon by former Interior Secretary Ken Salazar, a former Colorado senator.
"We're in a tough tough race here in Colorado because the Tea Party has decided this is Ground Zero for this election in 2014. We're not going to let the Tea Party take the United States Senate," Salazar said.
(Additional reporting by Gabriel Debenedetti in Washington; Editing by Caren Bohan and Steve Orlofsky)