A bipartisan coalition of two Washington Democratic state senators and 23 Republican senators took majority control this week.
The new majority voted Monday to form the caucus, saying one of the Democratic defectors, Rodney Tom, D-Medina, will be the new leader.
Democrats Tom and Tim Sheldon announced last month that they would form the power-sharing coalition with minority Republicans, creating a one-vote margin in the 49-seat chamber.
As part of the deal, Democrats and Republicans are supposed to chair six committees each and three committees would be split evenly and co-chaired by a member of each party.
Voters in November elected Democratic Gov. Jay Inslee, and the House is also controlled by Democrats.