CHEYENNE, Wyo. - Saying it's time for a new generation of leaders in Washington, Liz Cheney, daughter of former Vice President Dick Cheney, announced Tuesday that she will run against Wyoming's senior U.S. senator in next year's Republican primary.

Cheney is taking on popular Sen. Mike Enzi, who announced almost simultaneously on Tuesday his plans to seek a fourth six-year term.

Cheney's announcement is a political challenge unlike anything Wyoming has seen for years, maybe decades. Republicans in the state rarely challenge incumbents of their own party in national office. All three members of the state's congressional delegation and all statewide elected officials are Republican.

Liz Cheney, 46, is the elder of the two Cheney daughters. She is married with five children and was a resident of Virginia until recently. She and her husband bought a home last year in the posh northwest Wyoming community of Jackson Hole.

Asked why voters should oust a powerful incumbent in favor of a rookie, Cheney said seniority isn't necessarily an attribute.

"I think that part of the problem in Washington today is seniority. I think it's time for a new generation, for a new generation to come to the fore. I don't see seniority as a plus, frankly," Cheney told The Associated Press in a telephone interview.

Enzi made his campaign announcement more than six months earlier in the political cycle than he has in the past. He said he would continue to "do the job I was already elected to do."

"Working behind the scenes - this is what I have been doing since I was elected and this is what needs to be done," he said by email through a spokesman.

Enzi immediately won the endorsement of colleagues in the Senate, including the National Republican Senatorial Committee.

"Our support will be there for Mike," said the committee's chairman, Sen. Jerry Moran.

The race promises to be hard-fought. Enzi has had few serious Democratic challengers - much less Republican ones - since he was first elected to the Senate in 1996. He remains well-liked around the state as an affable former shoe salesman and mayor of the coal-mining city of Gillette.

Enzi, 69, takes pride in keeping a lower profile and remaining much less partisan than most of his colleagues. He often refers to his "80-20" rule - that opposing parties usually can agree on 80 percent of the details of any given issue - as a model for Republicans and Democrats to work together.

He handily won re-election in 2008 with more than 75 percent of the vote.

Cheney's interest in the seat has been an open secret for months, dating at least to last year's purchase of a home in Wilson, a community in Jackson Hole, that was listed for $1.9 million.

She appeared with her father at last year's state Republican Party convention. It was Dick Cheney's first public appearance since he underwent a heart transplant.

Since then, Liz Cheney has made frequent appearances at county-level Republican events in virtually every corner of the state. She also has been a Fox News political commentator.