Calling the Russian President an "autocrat in the Kremlin team", Michael McFaul accused Mr Trump and Mr Putin of teaming up to "trash an American".
Earlier this month, the President-elect tweeted to say: "Vladimir Putin said today about Hillary and Dems: 'In my opinion, it is humiliating. One must be able to lose with dignity.' So true!"
Citing Mr Trump's tweet, Mr McFaul said: "So now the President-elect and an autocrat in the Kremlin team up to trash an American. Unprecedented. Will Republicans speak up?"
Responding to questions about his tweet, Mr McFaul said: "I expect the leader of the free world to defend fellow Americans against gratuitous attacks from autocrats. I know I'm old-fashioned."
He added: "Why is Trump so militantly against an investigation into Putin's meddling in our elections? What does he have to hide? This love fest is odd.
"And the gall of Putin to be lecturing losing candidates how to act after a real election when he has never won a free and fair election ever."
Last week, Mr Putin criticised members of the US Democratic party for trying to “blame” Russia for Mr Trump’s election victory.
Speaking during his annual news conference to local and international journalists, he denied accusations of state-sponsored hacking during the campaign.
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