(CNN) When President Donald Trump admitted this weekend that in a July phone conversation with Ukrainian President Volodymyr Zelensky he did, in fact, raise "corruption" accusations against his 2020 political rival Joe Biden and his son, Hunter Biden -- he effectively turned up the volume on calls for his own impeachment.

"We don't want our people like Vice President Biden and his son creating to the corruption already in the Ukraine," Trump told reporters in describing the call. (There is no evidence of wrongdoing by either Joe or Hunter Biden.) And Monday, sources told CNN that White House officials are considering releasing a transcript of Trump's call with Zelensky.

Now House Intelligence Committee Chairman, Adam Schiff , normally reticent about impeachment, says that if a whistleblower complaint that was revealed last week shows Trump tying military aid to Ukraine to Ukraine probing the Bidens, "we may very well have crossed the Rubicon" on impeachment. Speaker of the House Nancy Pelosi spoke of "a grave new chapter in lawlessness."

Trump's administration has already intervened to keep secret the whistleblower complaint that appears to be related to Ukraine -- thereby blocking the normal process that would inform Congress of the controversy. And in doing so, he has aligned himself more closely with the tragic example of Richard Nixon, who resigned as Congress accused him of high crimes and misdemeanors.

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Trump made his regard for Nixon plain to me when, during a 2014 stroll around his office, he paused to pick up a framed letter. The note, signed in blue by the disgraced former President, urged Trump to go into politics. Trump told me he admired Nixon because he was such a good judge of talent.

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