WASHINGTON — After a particularly rough day for his campaign, Mayor Giuliani has lost two top supporters — one to the White House and one who could be headed to prison on charges of distributing cocaine. Mr. Giuliani’s top adviser in Iowa, Jim Nussle, is headed back to Washington after President Bush tapped him to lead to the Office of Management and Budget. A former congressman, Mr. Nussle served as the chairman of the House Budget Committee until January and made a failed bid for the Iowa statehouse last year.

Mr. Nussle was the first prominent Iowa Republican to join Mr. Giuliani’s campaign. His departure comes at a precarious time for the former New York mayor, who is trying to show his commitment to compete in the early-voting state after announcing that he would not participate in its traditional straw poll in August. Mr. Giuliani is making a speech in Des Moines today, and his top national advisers are spending the week in Iowa to build support and boost the campaign’s grassroots organization.

Also yesterday, the Giuliani campaign announced the departure of its South Carolina chairman, Thomas Ravenel, after a federal grand jury indicted him on charges of distributing cocaine. A former real estate developer, Mr. Ravenel was elected state treasurer last year. The Giuliani campaign issued a terse statement late yesterday.

“Our campaign has no information about the accusations pending against Mr. Ravenel,” the political director, Mark Campbell, said. “Mr. Ravenel has stepped down from his volunteer responsibilities with the campaign.”

The developments came on a day when the campaign was responding to a report in Newsday that Mr. Giuliani quit the Iraq Study Group last year after failing to show up for a single meeting. The report said Mr. Giuliani missed the meetings to give paid speeches and that his absences prompted the panel’s Republican co-chairman, James Baker, to ask him to either start showing up or leave the group.

“As someone considered a potential presidential candidate, the mayor didn’t want the group’s work to become a political football,” a campaign spokeswoman, Katie Levinson, said. “That, coupled with time constraints, led to his decision.”