That's a tricky game even in the often-mindless circus of American politics.  It can backfire quite easily if the attacks are seen to be way over the line, especially in a general election.  The Republicans launched vicious, xenophobic, racist attack ads on the Dems' views of immigration last year, especially in the Deep South, Midwest and Southwest -- the thought apparently being that Mexicans would trump the war, for some reason.  They backfired.  (Funny story: Turns out -- as I've been telling my fellow political junkies here in the states for years it would -- that Americans love to gripe about illegal immigration, but that, at the end of the day, they really don't give a shit.)  The Republicans looked like exactly what they are: vicious, xenophobic and racist, in addition to incompetent (already a given by that point).

Strategically, even if Royal could launch a massive attack, it would be highly risky to say that Sarko is dangerous -- the risk being that she would make it impossible for voters to take her seriously if they didn't perceive Sarko to be truly dangerous.  If you're going to call a candidate dangerous, you'd better have a reason, or you'd better make one up very quickly.  There had better be some serious evidence that he is either preparing to try to take over the world or having sex with his dog or something.

Be nice to America. Or we'll bring democracy to your country.