Is Political Correctness nothing more than "stealth" censorship?

Some comments about political correctness in another thread got me to thinking.

In the US, the First Amendment prohibits the government from censorship.

That notwithstanding, in the realm of national politics, the opprobrium associated being politically incorrect can be harsh to the point of being harmful.

For example, it became pretty clear from the start that it was politically incorrect to criticize President Obama. The consequence for being politically incorrect in this case was (less so now that he is a lame duck) to be stigmatized as a racist, a serious accusation and libelous if not true,

It is equally politically incorrect to talk about the reasons that Islam may be irreconcilably at odds with some of the basic foundations of the Western democracies. The stigma in this case is to be labeled as Islamophobic, in some cases by federal officials. Again, a serious and potentially libelous accusation.