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DENVER — Vice President Joe Biden is in town tonight to headline a fundraiser for Colorado Sen. Mark Udall, one of a number of vulnerable Democrats whose reelection bids could determine which party controls the Senate next year.

Udall, seeking a second six-year term, is facing a challenge from GOP Congressman Cory Gardner in what’s become one of the most closely-watched and competitive Senate races in the country.

Biden, who is giving the commencement address to the 2014 class of graduating cadets at the Air Force Academy in Colorado Springs on Wednesday morning, flew in early to appear at a joint fundraiser for Udall’s campaign and the Colorado Democratic Party.

After landing at Denver International Airport Tuesday afternoon, Biden rode in a motorcade along with Denver Mayor Michael Hancock and District Attorney Mitch Morrissey to Common Grounds Coffee Shop in Sunnyside, where he joined Udall, City Attorney Scott Martinez and other activists around a table to discuss the need to pass immigration reform.

“We have eleven million good people who need a good shot,” Biden said according to a pool report, noting the need to push Congress to act on legislation before the August recess, which seems unlikely given resistance in the GOP-controlled House.

“Now is the time to act. The one request I have is keep up the drum beat on these guys,” said Biden about House Republicans. “I believe in immigration reform with all I have.”

Members of the press were ushered out of the coffee shop before Udall and others spoke.

The fundraiser will take place later Tuesday night at a private home in Denver’s Hilltop neighborhood.

Udall will be in attendance, his campaign confirmed to FOX31 Denver.

Jill Biden, the vice president’s wife, is hosting a separate “Women For Udall” fundraiser on Wednesday morning in Colorado Springs.

Earlier this year, Udall made headlines when he couldn’t clearly answer a question live on CNN about whether or not he’d be willing to campaign alongside President Obama this fall.

Republicans across the country, including Gardner, are trying to make the 2014 midterm elections a referendum on the president, whose approval ratings is stuck around 40 percent, and on the Affordable Care Act.

In response to this FOX31 Denver report about the Biden fundraiser, Republicans fired away.

“Vice President Biden’s trip to Colorado is another vote of confidence from President Obama in Sen. Udall’s ability to carry out his agenda,” said Gardner’s campaign spokesman, Alex Siciliano. “While Senator Udall attempts to distance himself from President Obama publicly, he has been more than happy to vote with the president 99% of the time — and now Vice President Biden is coming to reward him financially.”

The RNC also took aim at Udall.

“It’s no surprise that after supporting President Obama 99 percent of the time that Mark Udall is being rewarded with a closed-door fundraiser with the Vice President,” said the RNC’s Michael Short. “But while Udall’s deciding vote for ObamaCare and continued obstruction of the Keystone Pipeline are paying dividends for his campaign account, Coloradans are the ones having to live with cancelled insurance plans, Medicare cuts, and unrealized jobs.”