From a Washington Post op-ed jointly authored by George Conway and former Acting Solicitor General Neal Katyal:
So it appears that the president might have used his official powers — in particular, perhaps the threat of withholding a quarter-billion dollars in military aid — to leverage a foreign government into helping him defeat a potential political opponent in the United States.
If Trump did that, it would be the ultimate impeachable act. Trump has already done more than enough to warrant impeachment and removal with his relentless attempts, on multiple fronts, to sabotage the counterintelligence and criminal investigation by then-special counsel Robert S. Mueller III and to conceal evidence of those attempts.
The president’s efforts were impeachable because, in committing those obstructive acts, he put his personal interests above the nation’s: He tried to stop an investigation into whether a hostile foreign power, Russia, tried to interfere with our democracy — simply because he seemed to find it personally embarrassing.