What is the legal loophole on martial law that allows the president to remain indefinite?

I don't know if that is really true. If it is, it must be really hard to do. Otherwise, Abe Lincoln would not have bothered with a re-election that even his own party thought he would lose. He wouldn't have gone to the lengths that he did to get re-elected (suppressing the press, ordering a m execution in Minnesota because, "I need those votes", putting fear into the opposing party by labeling anyone with a view contrary to his as a "Copperhead" and ordering their arrest and deportation, etc). Martial law was already in effect.