Can a US Congressman be impeached?

Answer 1

You probably can't.

Copied from Wikipedia:

The central question regarding the Constitutional dispute about the impeachment of members of the legislature is whether members of Congress are officers of the United States. The Constitution grants the House the power to impeach "The President, the Vice President, and all civil Officers of the United States." [3] It has been suggested that members of Congress are not officers of the United States.[5] Others, however, believe that members are civil officers and are subject to impeachment.[citation needed]

The House of Representatives did impeach a senator once:[6] Senator William Blount, in 1798. The Senate expelled Senator Blount and, after initially hearing his impeachment, dismissed the charges for lack of jurisdiction.[7] Left unsettled was the question whether members of Congress were civil officers of the United States.

Answer 2

If the president lies under oath or commits a crime like a felony.