(11-07) 09:25 PST SACRAMENTO -- California Democrats appear to have picked up a supermajority in both houses of the state Legislature Tuesday night, a surprise outcome that gives the party the ability to unilaterally raise taxes and leaves Republicans essentially irrelevant in Sacramento.

Democrats were long expected to gain a two-thirds advantage in the Senate, but Assembly Speaker John Pérez had downplayed expectations that the party could win a supermajority in the lower house. The party's apparent capture of 54 seats in the 80-member Assembly and 27 in the 40-member Senate would mark the first time in nearly 80 years that one party controlled two-thirds of both houses, according to Senate President pro tem Darrell Steinberg.

While several legislative races in swing districts are still close - including an Assembly race in the Central Valley and another in Orange County - both Pérez and Steinberg said Wednesday that they are confident Democrats' slim leads will hold.

But Paul Mitchell, a political expert whose Sacramento-based firm, Redistricting Partners, provides campaigns on both sides of the aisle with voter information, said it's unclear how many provisional and absentee ballots are still outstanding and that either of the Assembly races could still turn for Republicans. He cited in particular the Assembly race between Democrat Rudy Salas and Republican Pedro Rios in a district that spans Kings and Kern counties; Salas was leading this morning by just 300 votes.

A two-thirds majority would not only hand Democrats strong control of the executive and legislative branches, but give them far more power, including the ability to override vetoes by Democratic Gov. Jerry Brown, bypass legislative rules and deadlines and put constitutional reforms before voters.

When combined with the passage of Proposition 30, the governor's budget-balancing tax measure, the results offer lawmakers "a great opportunity to begin a new chapter in California," Steinberg said. He called a supermajority "a tremendous responsibility," and one that is "humbling to have."

"California is back on track ... we have come through this very tough period. When I started the deficit was $42 billion," he said. "Now we've made the awful cuts, and the voters have given us not only the tools to say no more cuts, but also to begin to focus on some positive agendas ... It will be very different to govern without a constant crisis."

The win for Democrats comes the first year that two major changes to election rules, both approved by voters, took effect in California: A new primary system, which allowed the top two voter-getters to proceed to the general election, regardless of their party; and the creation of a an independent citizens' commission to redraw Legislative and Congressional district lines, a change that made many races more competitive.

Democrats believe that a third change, the implementation of online voter registration - the system was rolled out in late September - was key for Democratic victories.

Both Steinberg and Pérez, however, pledged to continue to work with the GOP.

"The way one should govern is trying to bring everyone together," Pérez said. "Absolutely there is still a role for Republicans to express themselves through minority offices, through committees, etc."

Democratic consultant Steve Magivilio said Fullerton Mayor Sharon Quirk-Silva's apparent upset victory over incumbent GOP Assemblyman Chris Norby in Orange County put Democrats over the top in the lower house. She led by just over 1,000 votes early Wednesday.

"We had a good candidate and we just did very good, focused on communication with voters and we had a massive get out the vote effort," Pérez said of the Orange County district, adding that Democrats made a big push there in the final days of the campaign.

Democrats won big victories in the state Senate, including a decisive win by Assemblyman Bill Monning, D-Carmel, over his Republican challenger, Larry Beaman. Agoura Hills Democratic Sen. Fran Pavley was also holding onto her Los Angeles County seat, holding a 14,000 vote lead overRepublican Todd Zink, a deputy district attorney in the county. And Democrat Richard Roth was leading GOP Assemblyman Jeff Miller by more than 11,000 votes in a Riverside County senate district.

But in races that epitomize the changes instituted by the top-two primary and redistricting reforms, the speaker was losing his fight to keep at least one Democratic incumbent, and possibly two, in races that pitted sitting assembly members against challengers from the same party.

In Marin County, Assemblyman Michael Allen, D-Santa Rafael, was losing this morning to San Rafael Councilman Marc Levine, also a Democrat, in a race that attracted massive spending. And Assemblywoman Betsey Butler, D-Los Angeles, was down by 263 votes Wednesday morning in her race against Democratic Santa Monica Mayor Richard Bloom.

Pérez called Levine's apparent victory "a disappointing result."

"That is a loss, in my opinion, in terms of a great legislator and a great human being," he said.

San Francisco Chronicle staff writer Wyatt Buchanan contributed to this report.