From the site:
We the Tweeple of the United States, in order to form a more perfect government, establish communication, and promote transparency do hereby tweet the Congress of the United States of America.
We built this site to scratch our own itch. While searching for our local congressmen on Twitter we were amazed at how many folks on the Hill aren't tweeting. This site is a grass-roots effort to get our men and women in Congress to open up and have a real conversation with us.