Rob Portman, Director of the Office of Management and Budget, has resigned from his position after 18 months, in order to spend more time with his family.

The announcement was made Tuesday by President George W. Bush. He added that Portman would be replaced with former Iowa congressman Jim Nussle. Portman, 51, has been head of the Office of Management and Budget since April 2006 and will continue until August.

“There's no finer man in public service than Rob Portman. He's been a trusted adviser,” Bush said. “Fortunately, we've found a good man to succeed him.”

Portman’s surprise resignation comes less than three weeks after presidential counselor Dan Bartlett departed the White House, for family reasons.

The loss of one of President Bush’s most respected economic advisers comes at a particularly bad time, as the White House prepares to discuss next year’s budget with Congress.

Nussle, 46, whom Bush called “a strong advocate for fiscal discipline and a champion of tax cuts,” headed the House Budget Committee from 2001-06. He ran for governor of Iowa last year, unsuccessfully.

“Jim's name and knowledge command respect on Capitol Hill,” said President Bush.

Portman said he was leaving the White House because he wanted to spend more time with his family, who live in Ohio.