Is impeachment the only way to control a President who breaks the law? Can a President escape prosecution for criminal acts when in office? What of his subordinates, specifically members of his cabinet? If one is prosecuted can the President halt the prosecution or, failing that, announce that he'll pardon the accused in any event and then do so ? (I appreciate that Presidents do grant pardons on leaving office)

What prompts this is: Mr Blair has now been questioned twice by the police. They are investigating alleged corruption viz. that his government would grant honours e.g make someone a Lord, in return for substantial 'loans' to his Labour Party. The sums were loans only in that they were expressed as loans: there was no settled expectation that the money would be repaid. That was to avoid their having to be declared. Donations must be declared in accounts and publicly.The party's treasurer knew nothing of this activity(The loophole is to be closed). The man said to be the fixer in this, Lord Levy, was also questioned. Now Lord Levy has been arrested for conspiracy to pervert the course of justice. What that means is that he is accused of lying, or concealing evidence, to obstruct the police enquiry in some way e.g by tipping off witnesses or suspects or concocting some story for them. The Press are gleefully looking forward to Mr Blair himself being arrested.

If Mr Blair was arrested he would have to resign. He is not above prosecution: his office does not grant him exemption by virtue of his holding it. (Unlike the President of France who is completely immune from prosecution whilst in office. The current president is wanted for corruption and misuse of public funds when he was mayor of Paris. Because of the length of time he has served and is due to serve in office the prosecution will fail because it will be time barred by the time he leaves ) Only the Queen could grant a pardon on Mr Blair being convicted. She is supposed to take counsel of her ministers but she would certainly be advised by them that she should not (and she is highly unlikely to want to in any case )

The Long Answer from Straight Dope.