If Bush (Sr or Jr doesn't really matter) is to be put on trial for crimes (war or other ones) then so is the entire senate of the US & if either of the Bushes is to be put on trial, then so is Roosevelt, Reagan, Carter, Lincoln, Washington & every other Liberal or Conservative president the US has ever had, cause all of them have probably done things that are not lawdful, but as long as the majority of the Senate agrees & as long as a president doesn't use his veto right, they're free to do whatever they want.
(the right of an US president to use a veto & the possibility to veto in general is an entirely different discussion)