He was elected to four full terms in Congress after winning his first race in 1959. He resigned on September 9, 1968, to accept an appointment by Governor Nelson Rockefeller to fill the vacancy caused by the assassination of United States Senator Robert F. Kennedy on June 5, 1968. Having earned the support of both the Republican and Liberal parties in 1970, he lost in a three-way race to Conservative Party candidate James L. Buckley, having split the liberal vote with Democratic Party candidate Richard Ottinger.