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.) on Tuesday questioned President Trump for being "eerily silent" on the issue of Russian interference in the election, while arguing probes into the issue weren't meant to diminish his electoral victory.

"He's been eerily quiet about criticizing Russia. It's hard for me to understand. At every opportunity he's had to condemn the Russian involvement in the election, he has pivoted and not gone there," McCaskill told MSNBC's Greta Van Susteren.

The Democratic senator said the White House's response to the probe raises questions about possible collusion.

"Is there money here?" McCaskill asked. "Did he make some agreement? Is there a backchannel communication we don't know about?"

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"Maybe he just feels so defensive that this is somehow diminishing his victory, which, this isn't what this is," she added. "This is about a foreign government and cyber warfare on our democracy."

The White House has recently deferred to outside counsel on matters relating to the Russia investigation following the appointment of a special counsel last month to investigate the Trump campaign's ties to Moscow.

McCaskill argued Tuesday that it's the responsibility of the administration to stand up to Russia, even if it isn't politically convenient.

"Russia needs to get a message from us loud and clear: That you're not going to come and screw with our democracy," McCaskill said.

The Trump administration has pushed back on calls to punish Russia for interfering in the election, with Trump repeatedly calling investigations into possible collusion between his campaign and Russia "fake news" that was "put out" by Democrats.