Washington ? Top members of President Bush’s national security team are leaving in one of the earliest waves of departures from a second-term administration – nearly two years before Bush’s time ends.

As rancor in the nation rises over handling of the war in Iraq, at least 20 senior aides have either retired or resigned from important posts at the White House, Pentagon and State Department in the past six months.

Some have left for lucrative positions in the private sector. Some have gone to academic or charitable institutions. The latest was Deputy National Security Adviser J.D. Crouch, who spoke favorably of Bush’s policies as he announced he was leaving last week.

Turnover is normal as an administration nears its end, but “this is a high number,” said Paul Light, a professor of public service at New York University and an expert on government.

“You would expect to see vacancies arise as things wind down, but it’s about six months early for this kind of a mass exodus,” he said.

One reason may be that Vice President Dick Cheney will not run to succeed Bush in 2008, setting the stage for wholesale changes at all levels of government no matter who wins the election. Some officials, speaking only privately, say some people may be leaving to avoid being associated with the increasingly unpopular Iraq conflict.

About six in 10 Americans say the United States made a mistake in going to war in Iraq, according to recent AP-Ipsos polling.