Trump: reasonable and serious grounds to suspect refusal to reveal evidence (Tax Returns, other things) that likely indicate widespread violation of the Emoluments Clause and actual conflict of interest precluding continuing as president.
Sessions: plain evidence on the record of a serious crime: lying to Congress during confirmation hearing. Various excuses are extremely thin.
Remember, impeachment is the charge. They are presumed innocent in their trial before the Senate, if they don't have the intelligence and good grace to resign.
Thing is, we're not naive. This probably won't happen. But that DOES NOT MEAN Democrats should not be insisting on it. Indeed, they should say, we will not entertain any other business apart from national security emergencies until these matters are resolved.