Constitution, Article II, Section 4.
http://topics.law.cornell.edu/constitution/articleii
The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors.
The very first person impeached and tried by the Senate was William Blount, a Senator on January 11, 1799.
http://www.senate.gov/artandhistory/history/common/briefing/Senate_Impeachment_Role.htm#4
Blount had already been expelled by the Senate, but those in the House thought that that wasn't enough for the treasonous jerk. They wanted him barred from ever holding public office, a result only available via the impeachment process. The high and mighty Senators agreed that he was guilty as h*ll, but weren't willing to let anyone, not even the House, have authority over THEM They ruled that Senators were not subject to impeachment and the precedent has stood ever since. It probably applies to members of the House as well. The House has never tried to impeach its own although it has expelled a few. BTW, things turned out as the House feared. Blount went on to become Governor of Tennessee.