On Monday, Rep. Ernest Istook (R-Okla.) is scheduled to announce that he will give up his House seat to run for governor against popular Democratic incumbent Brad Henry.

“Unless there is a catastrophic change of mind, he’ll announce his candidacy on Monday,” a source close to Istook said Wednesday.

The already mobilizing field of Republicans to replace Istook in Oklahoma’s 5th district is expected to be deep. Because the seat is considered safely Republican, the victor of the GOP primary would likely win the general election.

At this point, Lt. Gov. Mary Fallin (R) is considered the frontrunner, followed by Oklahoma City Mayor Mick Cornett (R) and state Corporations Commissioners Denise Bode (R) and Jeff Cloud (R), who has close ties to former Rep. J.C. Watts (R).

Attorney Ryan Leonard (R), son-in-law of former Gov. Frank Keating (R), also is considering a run, as is state Rep. Kevin Calvey (R), according to Republican insiders based in Oklahoma.

“If Congressman Istook does announce for governor, I think it’s a strong possibility she’ll announce her candidacy for the 5th Congressional district,” Fallin’s chief of staff Nate Webb said in an interview.

Fallin is considered the leading candidate to replace Istook because she received more votes than he did in the 5th district during her statewide run for lieutenant governor in 2002, and because she has strong personal ties to the district — she’s a native of Pottawatomie County, located within its boundaries.

When it comes to voter enrollment, the 5th district is mostly Democratic, as is Oklahoma. But the seat is considered safely Republican because Oklahomans tend to vote GOP for national office, as opposed to their preference for Democrats when it comes to state and local office.

“We never want Members to leave. But if they do, we would prefer it be a seat like this,” said National Republican Congressional Committee spokesman Carl Forti.

Oklahoma Democrats disagree.

Buoyed by what they see as public dissatisfaction with Congressional Republicans in the wake of Hurricane Katrina and Rep. Tom DeLay (R-Texas) being forced to step down as House Majority Leader due to his indictment Wednesday on charges of conspiring to break Texas election laws, Democrats believe an open 5th district seat is eminently winnable.

“It’s a new ballgame, a new day for Democrats,” said Lisa Pryor, chairwoman of the Oklahoma Democratic Party. “We have strong leadership and strong ideas. The 5th district is certainly one that is competitive for Democrats, and we will not concede it by any means.”

Pryor said there has been interest in Istook’s seat from several highly qualified Democrats, but she declined to discuss their names.

According to sources, however, they include Bert Smith, who lost to Istook last year, former Lt. Gov. Jack Mildren, Oklahoma City businessman Jim Myers, and two unnamed state legislators, one a state representative and one a state senator.

Registered Democrats outnumber Republicans statewide by more than 278,000 voters, with 227,000 independents, and hold a lead of 10,600 voters in the 5th district, with 52,600 independents.

Nevertheless, President Bush won 66 percent of the statewide vote last year, scoring 64 percent of the vote in Istook’s district. Istook beat his Democratic challenger Smith by 22 percent.

Henry, meanwhile, narrowly edged out former Rep. Steve Largent (R) in 2002 to win his first term as governor, re-establishing the Democratic Party’s historical hold on that office.

Only three Republicans in state history have been elected governor.

Further signaling Istook’s uphill battle, a recent poll commissioned by the Oklahoma Republican Party found Henry popular, with a 64 percent to 25 percent favorable/unfavorable rating. And he led Istook in a hypothetical matchup, 44 percent to 36 percent.

But some Oklahoma Republicans say Henry owes part of his popularity to the fact that he rarely has been criticized, whether by Largent during his successful bid for office or by others since assuming the governorship.

Republicans also point to Istook, noting that as a state representative, he was given little chance to win the 5th district seat when he first ran in 1992.

Istook began to run for Oklahoma’s open Senate seat in 2004 but quickly bowed out when most of the state GOP establishment got behind Kirk Humphreys, who was then the mayor of Oklahoma City. Humphreys was swamped by former Rep. Tom Coburn (R-Okla.) in the Republican primary, and Coburn went on to win the seat handily.

Istook has just the kind of hubris it takes to win an uphill battle against Henry, one Oklahoma Republican insider said.

“Will I say the odds are he’ll win? No, I won’t. You have a popular Democratic governor who has no negatives, and the odds would certainly be against him winning. But that doesn’t mean he can’t,” this insider said.

“In his mind it’s not an obstacle to go against a giant, it doesn’t phase Ernest at all. He will be very aggressive.”