DENVER >> President Barack Obama urged Colorado Democrats to throw their support and their money behind the state’s Sen. Mark Udall, who is locked in a tight re-election bid. But the senator himself was nowhere to be seen, having decided at the last minute to stay in Washington rather than appear alongside the president.

Udall’s campaign said the senator needed to stay in Washington to cast his vote for Obama’s nominee to lead the Department of Housing and Urban Development, who was overwhelmingly confirmed. But Udall’s absence raised fresh questions about whether Obama is more of a hindrance or a help to vulnerable Democrats seeking re-election this fall.

Obama made no mention of Udall’s unexpected absence, but cast the senator’s re-election as crucial to helping him move forward on his agenda during the remaining years of his presidency.

"Mark Udall is a serious person who is trying to do the right thing," he told donors packed into a Denver hotel ballroom. "He’s not an ideologue. Doesn’t agree with me on everything. But he believes in the core idea that should be what Democrats are all about: the idea that if you work hard, you should be able to make it."

Republicans chortled at the news that Udall missed the fundraiser for his own campaign. Alex Siciliano, a spokesman for Rep. Cory Gardner, R-Colo., who is opposing Udall, said that now that the senator "has been called out on being a rubber stamp for President Obama’s agenda he has decided to hide in Washington, D.C., instead of face voters back in his home state. Senator Udall’s bizarre behavior these last few days will no doubt leave many Coloradans questioning his integrity, and rightly so."

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.

Tickets to Wednesday’s fundraiser ranged from $1,000 to $30,000, with the money split between Udall’s campaign and the Democratic Senatorial Campaign Committee. While Obama has attended numerous events for the DSCC, Wednesday’s luncheon marks the first time he has appeared directly on behalf of an endangered Democrat.

Before Wednesday’s schedule shake-up, Udall consultant Mike Stratton had said the campaign saw no reason to avoid a fundraiser with Obama.

"If the president is in town, and the president wants to be helpful, why would anyone want to pass that up?" he said. "I don’t think one visit by the president is going to change anything in terms of the dynamics of the campaign."

Ahead of the Udall fundraiser, Obama touted his administration’s efforts to boost jobs and economic growth during a speech in a Denver park. He said that while he understands that Washington looks "screwed up" to many Americans, there have been positive steps toward strengthening the economy.

"Despite what you may hear, there is no doubt that we are making progress," he said. "By almost every measure, we are better off than when I took office."

Still, he chided Republicans in Congress for blocking his agenda, accusing them of voting down "every serious idea to help the middle class." Taking on a sarcastic tone, he said the best thing that can be said about the GOP this year is that "they haven’t threatened to shut down the government."

The gridlock in Washington has contributed to Obama’s national approval rating slipping into the low 40s. But he remains a prolific fundraiser for Democrats.

With the Colorado Senate race expected to be one of the most expensive in the country, Udall wants to maintain a financial edge over his challenger. Gardner, who only announced his Senate bid in late February, had raised $3.7 million as of June 24, well short of Udall’s $12 million haul.

Even with a fundraising boost, Udall’s appearance appeared likely to create some awkwardness. Udall has tried to define himself as an independent-minded politician, touting his criticism of the Obama administration’s surveillance programs. In a television interview in January, Udall declined to say whether he wanted Obama to campaign for him.

Meanwhile, Republicans in Colorado have made tying Udall to Obama a core part of their strategy. And a glimpse at the president’s poll numbers in the state makes clear why. A recent Quinnipiac University Poll put Obama’s approval rating in Colorado at a measly 38 percent.

Obama advisers expect the president to do more campaigning for individual candidates as the election nears. But the White House’s broader strategy for the midterms has been to do no harm to vulnerable Democrats, even if that means Obama keeps his distance.

So far, that appears to be the preferred approach for most Democrats in tough races, including Alaska Sen. Mark Begich and North Carolina Sen. Kay Hagan.

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Associated Press writer Charles Babington in Washington contributed to this report.