Bill Berryhill's pitch to voters at a recent candidates forum came down to more than just his positions on issues affecting San Joaquin County's 5th Senate District.

It was about the balance of power in the Capitol.

"Do you realize if one party gets sole power, you think you're taxed to the max now? Guess what? You better grab your wallet, because they only have one way to go and that's to tax and spend, tax and spend," the Republican assemblyman told the crowd at Tracy's Kimball High.

Berryhill is running in one of a handful of newly drawn swing seats that will determine whether Democrats capture a supermajority in the upper house for the first time since 1965.

Senate Democrats need just two seats to hit the 27-vote threshold, and they have a good shot under the state's new political maps. The pick-up potential for Democrats this year is high enough that Republicans sought to have the districts thrown out via the courts and backed a referendum, which has qualified for the November ballot.

A sweep of the swing seats up for grabs this year could give Democrats a supermajority in the Senate through the better part of the decade.

Democrats also are two seats away from two-thirds in the Assembly, but experts doubt they can pick up those districts this year. With fewer opportunities based on the maps, observers say it's tough to handicap future races until the first run of the state's new primary system is complete.

For Democrats, supermajority control could put an end to years of often unsuccessful negotiations with the minority party over taxes, constitutional amendments and other issues that require a two-thirds vote.

"I'm going to lead the Senate and do everything we can without asking one Republican to give us one vote," Senate President Pro Tem Darrell Steinberg, D-Sacramento, told Democrats gathered at the state party convention in San Diego earlier this year. "And when they come in with their list of demands, we're going to tell them no such luck."

That prospect has Republicans, already marginalized in the budget process due to changes approved by voters in 2010, on high alert.

"It's a game changer because already the Democrats have majority of both houses, they've got the Governor's Office, they've got all the constitutional offices," Senate Republican leader Bob Huff said. "They clearly define the politics, they clearly define the agenda."

Though Republicans are at a registration disadvantage in every swing Senate district and generally trail Democrats in fundraising, Huff said he believes their message that a supermajority will lead to more taxes and get rid of checks and balances in Sacramento will keep his caucus above the one-third mark this year.

"If people are still voting their pocketbook, Republicans are going to win," he said.

While taxes will likely be in the spotlight in the targeted races, a supermajority wouldn't make an aye vote for new revenues a sure thing.

Democratic legislative leaders would also need to convince a growing bloc of business-friendly moderate Democrats and Democratic Gov. Jerry Brown, who pledged during his campaign not to raise taxes without a vote of the people.

The Assembly would still need GOP votes for taxes. But experts say a supermajority in the Senate would put the pressure on the Assembly to reach an accord, if not on new taxes then on agreements to eliminate tax breaks.

"If all you need is one or two members in the Assembly it becomes easier for Brown and the legislative Democrats to find something for (Republicans) to go along with the vote," said Wesley Hussey, an assistant professor of government at California State University, Sacramento.

The issue is already shaping the debate in the 5th Senate District, as Berryhill and San Joaquin County Supervisor Leroy Ornellas fight for a spot in the November runoff.

Ornellas, a Tracy Republican, recently signed the Americans for Tax Reform no-tax pledge, criticizing both Berryhill and Democrat Cathleen Galgiani, an assemblywoman from Stockton, for not doing the same.

Ornellas has also targeted Berryhill's support for legislation to require Amazon.com and other online retailers to collect sales taxes on purchases made by Californians, a vote Berryhill defends as a move to protect brick-and-mortar stores and bring jobs to the state.

Galgiani, who should easily make it into the runoff as the sole Democrat on the ballot, is keeping quiet on the issue. She skipped the recent forum where the two Republicans tangled on taxes and other issues, citing a scheduling conflict.

One mailer a committee funded by Realtors sent in support of her campaign doesn't even identify her as a Democrat. Instead it praises her as an "independent voice" who worked to protect homeowners affected by the mortgage meltdown and financial assistance for low-income college students.

Experts say Democrats will likely downplay the tax implications of the elections as they communicate with voters in districts across the state.

Jaime Regalado, former director of the Edmund G. "Pat" Brown Institute of Public Affairs at California State University, Los Angeles, said the Democrats' campaigns will be "talking more about a balanced approach," highlighting budget cuts they have had to make without new revenues.

"They won't be talking about taxes, or if they do, they won't be talking about taxes in a vacuum," he said.

Anti-tax activists say they'll work to make sure voters get the message loud and clear, focusing resources in the few swing districts.

"Those heavily Republican seats, we're not going to be looking at those," said Howard Jarvis Taxpayer Association President Jon Coupal, whose organization has not endorsed in the 5th Senate District race. "We'll be looking at those that are marginal."

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