Both are highly inappropriate terms. (Which is why I don't spell them out!), however, one is more wrong because of the message it sends and the intention behind it.
"Impeach the M****** F*******" does not mean what it sounds like. This is a phrase that was used in anger toward another person. It is not implying that the intended recipient is actually a MFer. Additionally, the term does not dehumanize the target.
"Grab them by the p*****" on the other hand, means exactly what it says. This is misogyny at its peak. It actually dehumanizes the individual referred to; it makes her an object. It is not a statement made in anger, it is a statement made from power. It places the speaker at a level above the target. It dehumanizes her.
Now, I am not a big fan of bad language, but if I see it, or hear it, I am going to put it in context.
The reason why we want to get old white men out of politics is because they have shown that they only care about their own power. This power needs to be taken away. Democracy is the beautiful thing that can take it away. We are trying to make them understand that they are not on a pedestal. That is where they place themselves at the moment. Historically, that is where they have placed themselves, as well. It is time that white men realize they are on the same level as everyone else.
This country is the land of the free. It is free for everyone. We do not have the right to dehumanize ANYONE who lives in it. We owe it to society to step into the shoes of the less fortunate and be a part of the coming change. It starts with avoiding verbiage that dehumanizes others - and being willing to call it what it is.