hilzoy: Normally, countries in which the head of state has this power are called "dictatorships".

Giuliani's no good, and Romney maybe not much better.

Crane says he was disappointed with Romney's answer to his question the other night. Crane asked if Romney believed the president should have the authority to arrest U.S. citizens with no review. Romney said he would want to hear the pros and cons from smart lawyers before he made up his mind. Crane said that he had asked Giuliani the same question a few weeks ago. The mayor [Giuliani] said that he would want to use this authority infrequently. (emphasis added)

I hope I don't have to explain why no human being should have this authority. If you don't already believe this, consider hilzoy's take at Obsidian Wings (I got the from her, and she got it from Glenn Greenwald at Salon).