https://stephenlendman.org/2021/01/can-a-former-us-president-be-impeached-and-convicted/

The Constitution is unclear on this issue. Nothing in the Constitution permits impeachment of a former president. Yet nothing rules it out.

The dems have time and again show abslutely no remorse for breaking the law. Even in this impeachment, there is no precedence where articles of impeachment against a sitting President have been drawn up and passed by House in 2 days, without any care about due process and without even putting the evidence on record, nor allowing President Trump to even respond to these crazy charges.

This utter lawlessness is not going to stop.

https://constitutioncenter.org/blog/can-president-trump-be-impeached-after-he-leaves-office

In December 2019, the Washington Post interviewed six scholars about that very question. Three believed it was a possible but unsettled question that a former president could face a Senate trial; two others said the Senate lacked such powers; and one scholar believed the Senate could try a former president.

There is precedent too:

Scholar Frank O. Bowman also pointed out another precedent: the 1876 impeachment trial of William Belknap, who served as Secretary of War for President Ulysses S. Grant. Belknap faced allegations of receiving kickbacks, and he resigned moments before the House approved articles of impeachment. The House charged Belknap with “basely prostituting his high office to his lust for private gain.” At Belknap’s trial, the Senate passed a motion in a 37 to 29 vote that “William W. Belknap, the respondent, is amenable to trial by impeachment for acts done as Secretary of War, notwithstanding his resignation of said office before he was impeached.” The Senate later acquitted Belknap on all charges, lacking a two-thirds majority to convict.