By Labor Day, Donald Trump will probably be under indictment and out on bail in four jurisdictions. In three of those (Florida, Georgia, and D.C.) he could be convicted of crimes that have prison as a possible penalty. In the New York case, falsifying business records is normally a misdemeanor but can be bumped up to a felony if the records were falsified to conceal a crime (e.g., violating election finance law). Just suppose Trump's chances of conviction and prison time were 50-50 in all four cases. That means he would have to win all four of them, which would have a probability of about 6%. This means that putting Trump in prison can't be dismissed as highly improbable.
How much time would he get? That is up to the judge, but there are federal and state guidelines as well as minimums and maximums in the laws themselves. There is also jurisprudence about similar cases in the past. We can't imagine him getting much, if any, prison time in New York. The crime isn't serious enough and Trump is a first-time offender. A fine is more likely there.
As to the Mar-a-Lago documents case, there are plenty of recent examples. In 2017, Reality Winner, who was working for a military contractor, leaked secret documents to the media. She was tried, convicted, and sentenced to 5 years in prison for this. In 2019, former contractor Harold Martin got 9 years for taking home classified documents. In June of this year, former analyst Kendra Kinsbury got 4 years for taking home documents about al-Qaeda and Osama bin Laden. Of course, the documents Trump took might have been more highly classified and his extreme sloppiness in handling them could affect a potential sentence.
As to the conspiracy charges, Stewart Rhodes got 18 years for his role in the coup attempt. Trump's role is surely greater than Rhodes', and Tanya Chutkan has a reputation for being a tough judge.
Suppose Trump gets a prison sentence. How would that work? All former presidents since Richard Nixon have been entitled by law to round-the-clock protection by the U.S. Secret Service for life. Could the Secret Service protect Trump if he were in prison? The USSS is beginning to think about it.
Protecting Trump while in prison changes the nature of the problem for the Secret Service. On the one hand, it is easier, since it is very rare for people to break into a prison. On the other hand, Trump would be in close contact with other prisoners, some of whom might not wish him well. He could obviously have a private cell, but what about meals, recreation, health care, and other things that would require him to be moved from his cell? Would prison authorities do that? Would the Secret Service do that? Would he be safe?
Various models are possible. At the low end, the judge might give him probation and tell him not to conspire to defraud the United States again. Next up is house arrest. Mar-a-Lago might be difficult to secure. Trump Tower might be easier if all the elevators but one could be programmed not to stop at Trump's floor ever and that one would require a key to make it stop there. The call button could be disabled. Armed guards could be posted at all the stairs. Alternatively, Trump could be confined to one of his other properties that was easier to secure. Or maybe the federal or state Bureau of Prisons could buy an easy-to-secure house somewhere for him.
Another possibility is a military base. Some of them are generally already fairly secure and if the base were large enough, a special facility with its own security could be built for Trump on base.
Continuing, Trump could be given his own wing of an existing prison, with his own cell, his own dining room, and his own recreation room, with Secret Service agents guarding the entrance to the wing. At the far extreme end is the 37-acre Supermax prison in Florence, CO, in the Rocky Mountains. Both the prison and the site are designed to handle hardened criminals with histories of violence. It could clearly do the job if need be. It would also be difficult for Trump's supporters to get there to riot.
It is also possible that the Secret Service could hand off all security duties to the Bureau of Prisons or some other agency. When Hillary Clinton was secretary of state, the Secret Service handed off her security to the State Dept. It can be done. The judge might even want to wade in on security before issuing the sentence, although that would be unusual. (V)