It depends on which part of US politics you are talking about.

Daily Kos (http://www.dailykos.com/) probably has the biggest readership of any American political blog, as well as one of the most active communities. But its influence extends only as far as the Democratic party, and probably only really the left wing of the Democratic party.

There isn't any blog that has the same influence on the GOP. A blog like Instapundit (http://www.instapundit.com) has a large readership and holds a lot of weight with conservative-leaning bloggers, but has nothing like the organized army of people you can find on Daily Kos.