Democrats can't name the specific action Trump should be impeached for

Assume for a second that Rep. Maxine Waters, D-Calif., is speaker of the House and that she’s agreed to go forward with the Democrats who want to immediately begin the impeachment process for President Trump. What is the specific crime or action they would cite to justify the proceeding?

Has anyone heard the exact crime or action that Trump engaged in that would justify impeachment? Democrats talk generally about “obstruction of justice,” or Trump’s “conduct,” but the specifics are curiously light, considering the gravity of what they propose.