Thankfully, Attempts on the Lives of Members of Congress are Rare
Thankfully, attacks on members of Congress are a rarity.
Yahoo details the basic history of assassination attempts on US Congressmen. Go here for details.
The last sitting member to be shot was Representative Leo Ryan, who was ambushed in Guyana in 1978. Before that, several Congressmen were shot, but not killed, by Puerto Rican nationalists who stormed the Capitol on 1954.
I think that the last member of the Congress to be shot and killed on US soil was Senator Bobby Kennedy, and that was more in the context of his candidacy for the Democratic nomination for president in 1968 rather than in his capacity as a legislator.
See, also, Salon: Assassinations are rare — but not unheard of.