Democratic Sens. Rodney Tom of Bellevue and Tim Sheldon of Potlatch said the bipartisan cooperation would drive better policies. Under the new plan, Republicans will chair six committees, including the panel that controls the state budget, while Democrats will control another six committees. The parties will split control of three other panels, though Sheldon is on two of those committees.
"This is not about power. This is not about control," said Tom, who will rise to serve as the new majority leader. "This is about governing in a collaborative manner."
Democrats have a small majority in the Senate, controlling 26 of 49 seats. With the moves by Tom and Sheldon, Republicans effectively hold a 25-24 advantage.
Along with sustainable budgets, the lawmakers said they want to promote job growth, reform the education system, and hold state government accountable.
Sen. Ed Murray, the Democratic leader in the chamber, said in a statement that he doesn't believe the Republicans' "take-it-or-leave-it plan" is the right way forward.
"We recognize that any majority in the Senate will be an unstable one, and we are committed to forming a mutually agreed-upon way for Republicans and Democrats to work together," Murray said.
Meanwhile, the chairman of the Washington State Democrats disowned the two defecting senators.
Dwight Pelz said he's long viewed Sheldon as a Republican, but the party had invested money to re-elect Tom this year. Pelz says that won't happen again, and the party will draft a candidate to oust him next time.
"This is a decision by Rodney Tom to switch parties back again," Pelz said. "Rodney Tom is a Republican now."
Tom was initially elected as a Republican but switched parties in 2006. Pelz said he believes the latest move was simply a way for Tom to fulfill his personal ambitions.
Senate Republican Leader Mark Schoesler said the new approach is the sort of cooperation the people of Washington and the country want to see.
"I look forward to showing you that the Senate can put politics aside and provide a responsible, bipartisan approach to the coming session," Schoesler said.
Democrats comfortably control the state House. The new legislative session begins in January.
One Republican committee chair is Sen. Pam Roach, who was kicked out of her caucus two years ago because of accusations of mistreating staff. She was allowed back in this year during the Republican budget coup last year, but she is still barred from interacting with Senate staff.
Mike Hoover, a senior Republican attorney for the Senate, had sued the chamber earlier this year and said he was subjected to a hostile and abusive workplace because of Roach. Under a settlement announced in September, the Senate reaffirmed its sanctions against Roach.
Tom said that would change, and the Senate committee that handles personnel matters would lift sanctions against Roach. He declined to assess how that decision could impact the lawsuit settlement, but he said Roach has vowed to run the committee appropriately.
"She's going to act in a professional manner," Tom said. "She has served her two-year period, and I think it's time to move on."
Hoover, who now works for the House, declined comment.
AP Writer Rachel La Corte contributed to this report.