What political parties are important in the US today?

Answer 1:

The three largest parties in the U.S. are:

Democrat

Republican

Libertarian

Note: The Democrat and Republican parties are by far in majority

Answer 2:

The Republicans and Democrats are still the two main parties in importance.

However, the Libertarian Party, the Green Party and the Independant Party are regarded as important in the sense of addressing ideas not getting enough attention by the two mainstream parties, and their ability to split votes.

So they will usually get mentions from the press during Presidential elections, and in some cases may even be invited to a debate.