Their caucus was Thursday night in Cranston. Mattiello received 44 votes; he needed 38.
Most of the 21 Democrats who didn't back Mattiello are women. Several said they felt that the legislative process isn't transparent and that Mattiello is dismissive of women's issues.
The Rhode Island National Organization for Women and the Rhode Island Coalition Against Gun Violence had asked lawmakers to oust Mattiello as speaker. Mattiello has also recently faced criticism over his handling of sexual harassment claims at the State House.
The speaker is officially elected by all 75 House members in January. With enough votes from Democrats, Mattiello is guaranteed the job because there are few Republican representatives.
Protesters who gathered outside the caucus at the Chapel Grille restaurant were dispersed by police for being on private property.
Democratic Rep. Charlene Lima said she supported Mattiello because he has "moved this state in the right direction as far as jobs, the economy and making it easier for businesses." She was chosen as part of his leadership team Thursday.
No one ran against Mattiello for speaker.
Mattiello said the vote shows that moderate and conservative Democrats are "solidly behind" him. He said he'll reach out to "progressives." He has called his opponents "extreme progressives."
"The story is, I've got enough votes to be speaker even with a coordinated attack on me," he said after the vote. "It was coordinated before my election and designed to effect the election."
Mattiello was narrowly re-elected in his Cranston district Tuesday.
Democratic Rep. Teresa Tanzi said it isn't about ideology, it's about leadership. The legislative process should be more open and transparent, she said, and the speaker shouldn't single-handedly control which bills come to the floor.
"Even if a majority of the members wanted something passed or a majority of committee members wanted it, he alone chose what lived and died. And that's impacting the entire state," she said. "One person should not be the sole gatekeeper, especially one person who was barely re-elected in his district."
Democratic Rep. Kathleen Fogarty read a statement outside the Chapel Grille on behalf of the dissenters. She said they are conservative, moderate and progressive Democrats, and they talked during the caucus about the "mockery of the committee process, the lack of transparency, the lack of trust and certainly the lack of respect." She said they've received no assurances anything will change.
"It's very telling that a lot of us here are women," added Democratic Rep. Moira Walsh. "Because as a general rule, the speaker of the House does not listen to the women vote."
Walsh talked about how Democratic Rep. Edie Ajello, a longtime abortion rights advocate, hasn't been able to get legislation passed to protect reproductive rights. Ajello also voted against Mattiello.
House Republicans are also looking for reforms to the way the House operates, said Republican Rep. Blake Filippi, the new House minority leader.
Mattiello said he's always willing to see if there's a way to do things better.
"We're going to continue to have an open door and work with people," he said. "And we'll put together as good of a process as we can."
Democrats in the Senate met for their caucus Thursday at the State House. Democratic Senate President Dominick Ruggerio was endorsed to return to his leadership role. The Senate Democratic caucus was open to reporters, according to the Senate spokesman. The House Democratic caucus was not.
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