Dornan's district, which was renumbered the 46th after the 1990 census, had always leaned Democratic, but became even more so when it absorbed a considerably larger number of Latino voters than he had previously represented. After failing to win the GOP presidential nomination in 1996, Dornan ran for reelection to the House against Loretta Sanchez, a former Republican who had switched parties and run as a Democrat. Dornan lost by 979 votes.
Following the narrow defeat, Dornan alleged that Sanchez's winning margin was provided by illegal voting from non-U.S. citizens. A thirteen-month House of Representatives investigation ensued, during which Sanchez was seated provisionally, pending the inquiry. A task force found 748 votes that had been cast illegally—624 from non-citizens in addition to 124 that had already been thrown out by California officials. This was not enough to overturn Sanchez' margin of victory and she was allowed to keep her seat. However, in consultation with the INS, the House committee identified as many as 4,700 questionable registration affidavits. The probe was dropped before these affidavits could be investigated.