Someone who describes herself as "conservative" would be

most likely to join:

The Republican Party

Explanation:

The Republican Party is one of the two main parties in the United States. It was founded on February 28, 1854. The Republican Party has represented different political ideologies in different eras, in the second half of the 20th century it became a socially conservative and economically liberal party.

Republican current voters represent all groups in American society, but statistics show that they are less supported by national and sexual and ethnic minorities; lower-income voters also tend to prefer the Democratic Party. The position of the Republican Party is considered particularly strong in Christian circles and in the southern states, and mainly outside the big cities of the nation.