Rep. Doug Ose (R-Calif.) announced Friday that he would not challenge Sen. Barbara Boxer (D) in 2004 and would honor his term-limits pledge by retiring from the House after the 108th Congress.

“I am writing you today to let you know that I will not run for the United States Senate, nor seek re-election to California’s 3rd Congressional District,” Ose said in an open letter to his supporters.

“While I may pursue public office again in the future, I have come to the conclusion that this is not my time to seek higher office. I did not make my decision due to a lack of support or financial commitments, but because of my strong desire to be a good husband and father.”

The 47-year-old lawmaker’s decision, which came as a surprise to Republicans in Washington and Sacramento, leaves the Republican Senate field essentially wide open while also setting up a potentially competitive GOP primary in the 3rd district.

Through the middle of last week, Republicans involved in Ose’s Senate exploratory committee fully expected him to jump into the Senate race at the end of May. Outside of his family, even those operatives closest to him did not learn of his decision until Thursday, according to sources.

While several names have been bandied about as potential challengers to Boxer, Ose’s effort was seen as the most advanced. He had been diligently raising money, touring the state and hiring operatives in anticipation of a race. Now, the situation is less clear.

“We understand and respect his decision,” said National Republican Senatorial Committee spokesman Dan Allen. “There are a lot of candidates that are looking at the race, and we believe that we will have a strong candidate once the field settles that will take on [Boxer] and make this race very competitive next year.”

Rep. George Radanovich (R-Calif.) is expected to announce his decision on the Senate race within the next few weeks. Before Ose’s announcement, most GOP observers thought it unlikely that Radanovich would run.

Beyond those two lawmakers, U.S. Treasurer Rosario Marin has been the most active possible GOP candidate, and she is continuing to discuss a run with Republican officials and potential supporters. State Assemblyman Abel Maldonado (R) could also emerge as a wild-card Senate candidate.

The 3rd district, meanwhile, has already been the scene of some political jockeying, as Ose was expected to vacate the seat regardless of whether he ran for the Senate.

The two most likely Republican hopefuls for the seat at this point are state Sen. Rico Oller and former state Attorney General, House Member and gubernatorial candidate Dan Lungren.

The Sacramento Bee reported last week that Oller has already lined up support from three conservative California Republicans — Reps. John Doolittle, Richard Pombo and Wally Herger.

Lungren would likely draw support from the more moderate wing of the state party, led by Rep. Bill Thomas. Lungren and Thomas were friends during their service together in the House and have remained close. The Bee reported that Lungren has put his home in the 4th district up for sale and is looking for property in Ose’s district.

The 3rd was made more Republican during the redistricting process, changing from a district that gave President Bush 52 percent of the vote in 2000 to one that would have given him 55 percent. The bulk of the seat now consists of three counties just east of Sacramento.

“If Bush got 55 percent in this district in California, good luck to the Democrats,” said Carl Forti, spokesman for the National Republican Congressional Committee.

But while no names have yet emerged on their side of the aisle, Democrats disputed the notion that this would be a walk for the GOP.

“Clearly it represents a potential opportunity for Democrats,” said Greg Speed, spokesman for the Democratic Congressional Campaign Committee. “California is trending in our direction. It’s also part of a growing retention problem for Republicans. We have potentially competitive open seats in Florida, Pennsylvania and now California.”

Ose was elected to the House in 1998 after working for several years as a Sacramento real estate developer. A moderate on many issues, the lawmaker has been best known on the Hill for his work on the Government Reform Committee.