The lawsuit, brought by Senator Marsha Blackburn of Tennessee, tests the reach of the Constitution’s “speech or debate” clause.

© Photographs by Kenny Holston/The New York Times

Mr. Smith’s testimony will be the latest public clash over the use of prosecutorial power in American politics.

A Republican eager to prove the former special counsel was dishonest appeared to instead demonstrate his own confusion over basic facts.

© Alex Kent/The New York Times