WASHINGTON, March 16 (Reuters) - President George W. Bush chose Idaho Gov. Dirk Kempthorne on Thursday as his choice to replace Gale Norton as Interior secretary, a Republican official said.

Bush was expected to make a formal announcement in the Oval Office at 5:30 p.m. EST (2230 GMT) on Thursday.

Kempthorne, who was first elected Idaho's governor in 1998, is also a former senator and one-time mayor of his state's capital, Boise. He had announced already that he was not seeking reelection as governor.

The Interior secretary job, which oversees federal lands, requires Senate confirmation.

Norton, the first woman to head the 156-year-old department and one of the original members of Bush's Cabinet, resigned last week after a tenure in which she often clashed with environmentalists.

Bruce Hamilton, national conservation director at the Sierra Club environmental group in San Francisco, was highly critical of Norton and said he did not think Kempthorne would be any friendlier to environmentalists' causes.

"Although he is known as a very nice, personable, noncombative person, he has an abysmal record on the environment," Hamilton said.

The Interior Department manages national parks, wildlife refuges and other federal lands, which account for 1 out of every 5 acres in the United States.

An unfinished item on Norton's agenda was the effort to convince Congress to allow drilling in Alaska's Arctic National Wildlife Refuge, still a priority for Bush.

Republicans are trying to push that plan through the Senate by attaching it to a budget now under debate. But that idea is controversial among Democrats and many moderate Republicans and faces its stiffest resistance in the House of Representatives.