(CNN) - Vice President Joe Biden closely aligned himself Monday with Democratic Sen. Claire McCaskill of Missouri, saying he's a "strong, strong ally" of the first-term senator.
"She knows who she is and what she believes in, and she's willing to take risks to fight for what she believes in," Biden said at a private fund-raising event in Kansas City, according to a pool reporter in the room.
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McCaskill made headlines last month when she announced she would not be attending the Democratic National Convention in North Carolina this September, raising speculation that she was attempting to distance herself from the president during her re-election battle.
The senator, however, strongly pushed back, saying she was simply trying to focus on the race.
"I've got a really hard election," McCaskill told CNN in June. "If you had a really hard election and it was after Labor Day would you go to North Carolina to a bunch of parties and glad-handing or would you stay home and work as hard as you know how to convince Missourians they should rehire you?"
At the event Monday, Biden said McCaskill was the "toughest, most principled person in the United States Senate."
Republicans have been aggressively targeting the first-term senator as a chance to pick up a Senate seat. Crossroads GPS, a conservative group founded by Karl Rove, has dropped hundreds of thousands of dollars on ads against McCaskill.
At least three Republicans are in the race to challenge McCaskill, and will face off in a primary on August 7. A national tea party group, the Tea Party Express, is backing Republican challenger and former Missouri State Treasurer Sarah Steelman. Also in the race are 6-term Rep. Todd Akin and the former chief of Germ-X, John Brunner.
The non-partisan Cook and Rothenberg political analysts rate the race as a toss-up, with Rothenberg noting it as leaning Republican.
- CNN's Ted Barrett and Gregory Wallace contributed to this report.