Washington - A Republican has resigned as the party's fourth-ranking member of the Senate after admitting to an extra- marital affair with an aide, US media reported Wednesday.

Senator John Ensign, who represents the state of Nevada, quit as chairman of the Republican Policy Committee, which coordinates legislative policy in the Senate, according to the Congressional Quarterly.

Ensign on Tuesday admitted having an affair last year with a campaign staff member who was married to one of his Senate aides.

The Nevada senator has been billed as a potential Republican presidential candidate in 2012.(dpa)