Who is ‘Sarah Palin impersonator’?

Solution 1:

The "Sarah Palin impersonator" refers to an actual Sarah Palin impersonator, a person who showed up at the meeting impersonating Sarah Palin. This person gained their 15 minutes of fame on broadcast television, but was otherwise not mentioned by the news media.

Solution 2:

Dour High Arch has this one correct. It was an actual, literal Sarah Palin impersonator: someone who got made up to look like Sarah Palin.

Oishi-san, this might be a good time to review the definition of impersonator:

impersonator: someone who pretends to be (another person) as entertainment or in order to deceive someone

In this case, entertainment might have been the object, but the intent was certainly to deceive. The article doesn't mean the other Reupblican candidates were mimicking the style of Sarah Palin, it means one person was actually pretending to be Sarah Palin.

Note that the article says

The Sarah Palin impersonator has left the building.

This is meant to echo the famous line:

Elvis has left the building.

which used to be announced at the end of Elvis Presley concerts so that the screaming fans would stop chanting for him to return and give yet another encore. The reason the article uses this construction is to emphasize the fact that the CPAC is officially over and everyone can go home and stop pretending something is still happening there. Short version: no more news is coming out of the CPAC, folks. Let's move on.

Solution 3:

It sounds to me like the author is speaking metaphorically. The Conservative Political Action Conference itself impersonated the style of Sarah Palin.

It can be simply many republicans who are followers of Sara Palin's credos and her action patterns. But in that case, 'inpersonator' should be in plural form. Am I wrong?

Since it was a metaphor, there was not a specific "impersonator" or "impersonators" referred to. It makes sense to speak in the singular.

EDIT: It seems there was an actual impersonator there, so the statement wasn't necessarily metaphorical. Good catch Dour High Arch.