James Shields served as US Senator from Illinois from March 5, 1849 to March 15, 1849 (sic) and again from October 27, 1849 to March 3, 1855. His first election was invalid: he had not been a U.S. citizen for the nine years required by The Constitution, having been naturalized October 21, 1840 and elected on January 13, 1849. He resigned from the Senate on March 14, 1849, returned to Illinois, campaigned once again for the seat he had resigned, and won a special election when he had been a citizen for nine years, replacing himself as Senator.

This is a precedent in the case of Rep. George Santos, should it turn out that he hasn’t met the constitutional citizenship requirement.