Thanks to the voter-approved independent redistricting panel, Democrats are close to achieving the supermajority they need to act without the support of Republicans, who have slipped to just 30 percent of registered voters statewide.

One of the key races for them to achieve that is the 5th Senate District, a moderate district that runs down the tip of the San Joaquin Valley from Galt to Modesto and is about equally divided between Republicans and Democrats.

Two members of the state Assembly, Republican Bill Berryhill and Democrat Cathleen Galgiani, tout their ability to cooperate across party lines even as they differ on issues such as funding the $68 billion high-speed rail system favored by Gov. Jerry Brown and a year-old law that transfers responsibility for lower-level criminals from state prisons to county jails.

Galgiani supports both, while Berryhill is opposed.

Democrats are counting on a win by Galgiani to gain their supermajority, even as she downplays the significance.

“I’m probably the most independent Democrat running for the Senate,” she said. “I’m one of those who works both sides of the aisle.”

But Berryhill said the possibility that a Galgiani win could be the key to bringing Democrats closer to supermajority in Sacramento is an attention-grabber for the district’s voters.

“That scares a lot of people,” he said. “It’s almost virtually giving a dictatorship to one party.”

Even if the Democrats did reach the threshold in the Senate, their prospects for doing the same in the Assembly are distant, so they would still need to find some Republican support.

Yet the ability for the November election to bring the Democrats a two-thirds majority in at least one house of the Legislature is the dominant dynamic of this year’s state legislative races. Democrats are two seats shy in the Assembly and Senate, but are seen as having a better shot of achieving that in the Senate.

If they succeed, it would be the first time that one party has had a supermajority in either house of theLegislature since California voters passed Proposition 13 in 1978, which raised the vote threshold to pass tax increases to two-thirds.

The last time either house had a supermajority was the 1977-78 session, when Democrats held a 57-23 advantage in the Assembly, said E. Dotson Wilson, the chamber’s chief clerk.

A supermajority would allow Democrats to approve tax increases, pass emergency legislation, reject gubernatorial vetoes or change house rules while ignoring Republicans.

Democrats could alter the state’s tax system in a way that currently can be blocked by minority Republicans, said Senate President Pro Tem Darrell Steinberg, D-Sacramento. He said that could include broadening the tax base while lowering tax rates and creating incentives for high-wage manufacturers to build in California.

That’s the sort of scenario that concerns Republicans, but they expect it also will resonate with voters who are unwilling to cede control to a Democratic Legislature and governor.

“We’ve got our base lathered up by, yet again, tax increases on the ballot,” said Senate Minority Leader Bob Huff, R-Diamond Bar. “Republicans are seen as the adults in the room who can make the tough choices and keep things running smoothly.”

Democratic Gov. Jerry Brown’s Proposition 30 would raise sales and income taxes to avoid deeper cuts to schools and other programs. Proposition 38, backed by wealthy civil rights attorney Molly Munger and thestate PTA, would devote an income tax increase directly to schools.

Whether the threat of higher taxes can stall the Democrats’ march to a two-thirds majority is unclear in a statewhere Republicans have been losing support quickly. Democrats also are focusing on the 31st Senate District in Riverside County and the 27th Senate District in Los Angeles and Ventura counties.

Democrats have a voter registration edge of 7 percentage points in the 27th District but are tied with Republicans in the 31st.

“When you get down to a 30 percent Republican registration statewide, it doesn’t matter how you draw the lines,” said Allan Hoffenblum, publisher of the California Target Book, which analyzes legislative and congressional campaigns. “The problem is that there are so few Republicans.”

Thanks to the state’s other major political reform, the new top-two primary system, candidates from the same party are running against each other in several general election races, a dynamic that has led to some of the most negative campaigning this year. Of the 100 state legislative races this year, 20 are same-party runoffs.

An illustration of how the new primary system has affected the general election can be found in Riverside County’s 67th Assembly District, where two Republicans are engaged in a lively campaign.

Melissa Melendez’s campaign website calls fellow Republican Phil Paule a “political ‘lap dog’” and “tool for big government.” At a campaign rally last month, she urged supporters to “choose between leadership and lawlessness” while contrasting her “strong record” with what she described as Paule’s criminal record — a decade-old drunken driving conviction.

“If we’re going to elect someone to make laws for the state of California, that person should also be able to follow the laws of the state of California,” the Lake Elsinore city councilwoman said in a telephone interview.

Dave Gilliard, Paule’s political consultant, said the personal attacks are backfiring with voters.

Paule counters that Melendez is siding with Los Angeles Mayor Antonio Villaraigosa over control of Ontario International Airport, a major issue in the economically struggling Inland Empire. Melendez said the dispute could lead to higher airport taxes and fees if elected officials aren’t careful.

“We can talk about lap dogs, we can talk about 10-year-old arrests,” said Paule, a district aide to U.S. Rep. Darrell Issa. “She hasn’t explained why she voted against the interests of her own constituency.”

In explaining the intraparty campaign attacks, Paule said, “It’s the new rules and so you play by the rules.”

In suburban Sacramento, Assembly Minority Leader Connie Conway, R-Tulare, took the unusual step of asking donors to drop Republican attorney Andy Pugno and support the party’s endorsed candidate, incumbent Assemblywoman Beth Gaines, R-Roseville.

Pugno is best known as the attorney who represented supporters of the 2008 Proposition 8 ban on same-sex marriages. He has not been actively campaigning.

While Republicans sort through seven same-party Assembly squabbles, Democrats have 11 of their own in the Assembly and two in the Senate.

That is forcing Speaker John Perez, D-Los Angeles, to devote time and money to defend incumbents Michael Allen, D-Santa Rosa, in the 10th Assembly District and Betsy Butler, D-Marina del Rey, in the 50th.

“Both of those are races that should have been over in June and would have been over in June under any other circumstance,” Perez said, referring to the top-two primary system. “But we’ve got a protracted battle all the way to November.”