A Democrat, Larry LaRocco, is also running for the seat in 2008, but political analysts say they expect a Republican, whether it is the one Mr. Otter appoints or someone else, to win a full term next fall in this conservative state.

Image Senator Larry E. Craig, Republican of Idaho, in Washington last year. Credit... Charles Dharapak/Associated Press

If he had run, Mr. Craig was widely expected to win another term, but that was before this week.

Since Monday, national Republicans, fearful that the party would be damaged in the 2008 elections by yet another prolonged scandal, had led a concentrated, relentless effort to pressure Mr. Craig to abruptly end a 27-year career in Washington that included a decade in the House and nearly three terms in the Senate.

Mr. Craig had been a powerful voice on many Republican issues in the West, fighting Democrats on issues ranging from gun control to proposals to protect salmon whose migration is blocked by dams that provide hydroelectric power and water to Idaho and other states. He pushed to expand nuclear energy projects in Idaho and elsewhere but also to provide federal funding to rural areas hurt by declines in federal timber harvesting, funding that was sharply cut this year. He also supported measures to restrict abortion and ban same-sex marriage.

On Tuesday, Senate Republican leaders stripped Mr. Craig of the committee leadership positions that helped make him influential. Mitch McConnell, the Senate Republican leader, who outmaneuvered Mr. Craig for the position of Republican whip in 2002, called his conduct “unforgivable.”

While Republican leaders in Idaho were far more measured in their reaction, few defended Mr. Craig and many state newspapers called for his resignation. The senator was lampooned on local talk radio shows, which aired excepts of an interview in which Mr. Craig called former President Bill Clinton a “naughty boy” amid the Monica Lewinsky scandal. At the same time many people, including some Democratic voters, said they were genuinely saddened by the turn of events.

Until today, Mr. Craig had said nothing publicly beyond a brief statement on Tuesday in which he insisted he was innocent and said he had made a mistake in pleading guilty.