WASHINGTON – Sen. John Ensign, R-Nev., once seen as a rising star in the Republican Party, Tuesday acknowledged an extramarital affair with a former campaign staffer who is married to one of the lawmaker’s former legislative aides.

Ensign, a member of the Senate Republican leadership, disclosed the affair at a hastily arranged news briefing in Las Vegas, his hometown. He flew home Tuesday after informing GOP leaders on Capitol Hill of his impending announcement, missing a vote on tourism legislation considered important to Nevada’s casino industry.

The news was the latest setback for a Republican Party that suffered losses of at least 13 Senate seats in the past two elections and saw Sen. Arlen Specter of Pennsylvania defect to the Democrats in April. Any further instability in their ranks is unwelcome news for Republicans, who viewed Ensign as a telegenic communicator who could deliver the conservative message on political talk shows in a congenial matter.

Ensign did not comment about his political future during his news conference. He does not face re-election until 2012 and had taken preliminary steps to explore a run for the White House that year, making a recent trip to Iowa, the first testing ground on the presidential primary calendar.

“I deeply regret and am very sorry for my actions,” Ensign said, reading from a prepared statement and leaving without taking questions. Ensign’s wife, Darlene, was not at her husband’s side during the short briefing but issued a statement saying the couple’s marriage has become “stronger” after the affair.