This is the name given to a member of a legislative body whose job is make sure his political party stays unified when debating and voting on bills.

Alrighty, so, here's what I'm thinking:
The legislative branch is made up of two different things, the House of Representatives and the Senate. 
In the House, the person in charge is the Speaker of the House.
In the Senate, the person at the top is the Vice President, although he really does nothing except in the case of a tie between all members of the Senate.
Therefore, I think the answer to your question is the Speaker of the House.

Hope this helps!

Louis xvi wanted to force aristocrats to pay taxes for the first time