Yes, the Senate can try Trump

For the Founders, it would have been obvious that the “power to impeach” included the ability to hold former officials to account. The impeachment power was imported to America from England, where Parliament impeached only two men during the 18th century, both former officers. No U.S. state constitution limited impeachments to sitting officers, and some allowed impeachment only of former officers. In 1781 the Virginia General Assembly subjected Thomas Jefferson to an impeachment inquiry after he completed his term as governor.

Why would former officers be included within the impeachment power? Impeachment trials had long served as a vehicle for exposing and formally condemning official wrongdoing, or for a former officeholder to clear his name. Disqualification from future office was also an important penalty. A former Vermont lawmaker was impeached and disqualified from future state office for leading one of the tax rebellions that spurred the drafting of the U.S. Constitution. The American founders understood the history of demagogues and dictators corrupting republics and the need to exclude them from future office. As one delegate to a state ratifying convention put it, men who held public office should be “within the reach of responsibility” so that “they cannot forget that their political existence depends upon their good behavior.”