How can something like mccarthyism be used as a partisan weapon against another political party? give an example.

McCarthyism is nothing more than a witch hunt.  A lot of finger pointing and not a lot of proof.  Both parties (Democrat & Republican) in the United States use this in todays world.  Ill give examples of both and follow up with how it can be beneficial to each party.

Democrats:  Accuse other politicians of being "racist" or "bigoted" just from political ideas and from certain members of the base.  While it is not fact or true that Republicans are by policy racist, it is a word that is hated by people and has a negative connotation to it, forcing some to keep distance from said person

Republicans:  Accuse other politicians of being "muslin lovers" or "muslins" themselves.  We saw this for the entire Obama presidency.  Congress and some Republican supporters would use the word "muslin" to describe the president in order to give a negative connotation towards Obama.  This has some strong effectiveness due to the recent events (September 11th, 2001).

Both parties are trying to stick a negative idea/precedent/description about the opposition in order to sway votes.  This tactic is very effective because  not only will you sway votes, theres little repercussion in doing so because the people who disagree with you are not going to be swayed, but that voter in the middle who cares about one issue over the other (in this case racism over fear of muslims or vice versa fear of muslims over racism).  

Either someone is intelligent enough to know the rhetoric between the two parties and votes by policy (unaffected / no positive or negative response), they don't care about either issue (unaffected / no positive or negative response), or someone is strongly in favor of one or the other (strong positive or negative response).

While there are some attempts that have been made and can be made that would be so egregious that most people have a negative response, but that rarely happens and would be deemed political suicide.  

Hope this helps.