https://www.charlestonmercury.com/single-post/a-senate-trial-of-president-would-be-unconstitutional
Those arguing for the power of the Senate to try a former president bring up an 1870s case of the impeachment of Secretary of War William Belknap. This impeachment came after his resignation but set bad a bad precedent in a unique case. The constitutionality of impeachment and trial was questioned at the time and was never ruled constitutional by the Supreme Court. Importantly, Belknap resigned hours before he was going to be impeached, solely to avoid impeachment. The evidence and witnesses against Belknap were beyond dispute. Belknap had even admitted to taking substantial sums of government money illegally from the profits of Fort Sill trading posts. Regardless of the evidence, a number of House members questioned the constitutionality of this impeachment. He was impeached anyway.
The Senate acquitted Belknap despite the overwhelming evidence and slam dunk case. Importantly, senators never doubted Belknap’s guilt.
The reason for his acquittal was that many senators held they didn’t have the power to try the case. Before the trial started, almost half the senators voted against the trial on constitutional grounds.