Many members of Congress will leave politics and become a as a profession?

Lobbyist

Explanation:

Becoming a lobbyist is a preferred choice for a lot of members of Congress when they leave office due to that profession offering great salaries especially for members of Congress that were active in Committees and other forms of politics.

The reason being that these ones that gained a lot of connections and so can help lobby for different issues better by simply approaching old colleagues or going indirectly through their contacts.