OLYMPIA — After months of indecision, Republican Dino Rossi plans to jump into the race to unseat Democratic Sen. Patty Murray this week, a Republican official said Monday.

The entry of the two-time candidate for governor transforms what could have been a walkover for Murray into a competitive race that will likely force her to spend her war chest here rather than helping vulnerable Democrats elsewhere.

The official said that Rossi is hiring staff and is preparing to make his campaign official on Wednesday. The official spoke on the condition of anonymity to discuss private campaign plans.

Republicans had been courting Rossi to get into the race, while national Democrats have been criticizing him in hopes of keeping him out.

But Rossi has been in no rush to start campaigning ahead of the Aug. 17 primary. The filing deadline for all candidates in Washington state is June 11.

Last week, former Alaska Gov. Sarah Palin endorsed another GOP candidate, former football player Clint Didier, a favorite of many tea party activists. But because of the top-two primary — a wide-open race that allows the two candidates with the most votes, regardless of party, to move forward to the general election — Rossi figures to advance.

The late start does mean Rossi will have to aggressively tap his supporters to raise the money needed to mount a viable campaign against Murray, who won a third term easily six years ago.

Republican political consultant Chris Vance said that in any other year, a candidate starting this late would be in trouble.

This year may be different because of a perceived anti-incumbent wave that is likely to help Republicans, Vance said.

“If the wave is big, Dino Rossi is going to win. If the wave shrinks, he’s probably not going to win,” Vance said.

On Monday, the Washington Poll, released through the University of Washington, showed Murray with a 44 percent to 40 percent advantage over Rossi in a hypothetical November showdown. The statewide telephone survey of 626 registered voters was conducted May 3-23 and has a margin of error of plus or minus 3.9 percentage points.

Rossi is best known politically as a runner-up, losing governor’s races to Democrat Chris Gregoire by a handful of votes in 2004 and a wider margin in 2008.

Rossi did not respond to phone calls or an e-mail seeking comment. His spokeswoman, Mary Lane Strow, refused to confirm or deny Rossi’s entrance into the race, but said he would make a statement midweek.

Murray was first elected to the Senate in 1992’s Year of the Woman. She ran that race as an unassuming “mom in tennis shoes” but has since climbed to the No. 4 spot on the Democratic leadership team. She faced a relatively weak Republican field before Rossi entered the race and had almost $6 million in her campaign account as of March 31.

Murray has aided powerful homegrown interests such as the Boeing Co., the state’s largest private employer. She also has delivered federal spending to all corners of the state and is known for a special focus on veterans’ issues.

Democrats have been anticipating a Rossi campaign and began attacking his background in real-estate investment weeks before he officially entered the race.

“We know that Patty’s got to work very hard, and there’s discontent in the land,” State Democratic Party Chairman Dwight Pelz said. “But they picked the wrong candidate to tap into that discontent.”

Among the dozen Republicans who have already announced plans to challenge Murray are two small-government conservatives: Didier, an eastern Washington farmer who formerly played football for the Washington Redskins, and state Sen. Don Benton of Vancouver.

Benton welcomed Rossi to the race.

“I look forward to debating him on the issues confronting us today,” he said. “I still believe that I am the best candidate to beat Patty Murray in November.”

Didier did not respond to phone calls or e-mails seeking comment.

As a state senator in 2003, Rossi worked with Democrats to balance a yawning budget deficit without general tax increases — a portfolio he is sure to reference heavily on the campaign trail.

A Roman Catholic, Rossi is personally opposed to abortion but has finessed the issue in previous statewide runs by pointing out that abortion law is largely a settled matter in Democratic-leaning Washington state. A Senate run will test that issue in a new way, since senators must confirm nominees to the U.S. Supreme Court.