"It is unlikely that Congress has the power to impeach a sitting justice for alleged offenses he may or may not have committed while a private citizen and a teenager. It would be the first time in American history any impeachment went back that far and focused on such adolescent conduct.

Democrats may try to move it forward by alleging that the grounds for impeachment include perjury committed by nominee Kavanaugh in his testimony at his Senate Judiciary Committee confirmation hearing. Before his Supreme Court confirmation Kavanaugh was a sitting judge on the U.S. Circuit of Appeals for the District of Columbia subject to the impeachment power of Congress.

But that would be a ploy, somewhat akin to the phony perjury grounds used to impeach President Clinton. Democrats were outraged when Republicans used perjury as a surrogate for sexual improprieties. They should be similarly outraged if their colleagues seek to use perjury as a pretext for religitating old charges of sexual misconduct against Kavanaugh."
By Alan Dershowitz