At the urging of party activists and officials, Cantwell formed an exploratory committee in October 1999 to consider a run for United States Senate against Democrat Deborah Senn and incumbent Republican Slade Gorton. She committed to run for the position on January 19, 2000. Cantwell entered the campaign a year after Senn; she quickly lost the endorsements by the Washington State Labor Council and NARAL to Senn.