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Word of Mr. Obama’s appearance in Colorado has already energized state and national Republicans. Brook Hougesen, spokeswoman for National Republican Senatorial Committee, called Mr. Udall “a loyal rubber-stamp” for Mr. Obama.

“Weeks after Vice President Biden came to Colorado to reward Sen. Udall, President Obama is coming to raise money for him and say thank you himself,” Gardner campaign spokesman Alex Siciliano said. “Senator Udall votes with President Obama 99 percent of the time; Cory has yet to meet one person in Colorado who agrees with the president that often.”

Mr. Biden appeared in Colorado with Mr. Udall at a May 28 fundraiser before heading to San Francisco for a fundraiser at the Steyer home.