Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.) said Monday that Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) is molding front-runner Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE into the best possible Democratic presidential candidate.

“Bernie’s challenge has made Hillary a better candidate,” she told host Chris Cuomo on CNN’s “New Day.”

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“It has made her stronger and more aggressive,” McCaskill added. "I have said an awful lot of good things about Bernie Sanders and the good work he is doing for America.”

McCaskill said she is endorsing Clinton rather than Sanders because the Vermont lawmaker "has not identified as a Democrat."

"He identifies as a socialist," she added. "The problem is in a state like mine it is very hard for voters to get past that."

McCaskill additionally said that Clinton has the advantage in a general election, given policies traditionally proposed by Republicans.

“I think she broadens the tent by highlighting the contrast between her policies and the Republican policies,” she said.

“Once the Republicans figure out who their nominee is, the contrast will be very clear,” McCaskill added. "I don’t think voters want to go to the failed, trickle-down economics that have frankly never worked.”

Clinton leads Sanders by 22 points nationwide, according to the latest RealClearPolitics average of polls.

He is proving competitive with the former first lady in the early voting state of New Hampshire, however, where he is currently edging her out by over 1 point.