SACRAMENTO (AP) — The California Supreme Court on Friday upheld a new political map drawn by the state's independent redistricting commission in a ruling that gives Democrats their long-awaited opportunity to control two-thirds of a legislative chamber, the threshold needed to pass tax increases.

The high court announced its unanimous decision that the Senate map drawn last year by the California Citizens Redistricting Commission should be used in the June primary and November general election, even though a Republican-backed referendum challenging the map is expected to qualify for the ballot.

Republicans, who challenged the map in a fight to maintain their influence in the Legislature, were dealt a major blow: No party has controlled two-thirds of either the Senate or Assembly since voters passed Proposition 13 in 1978 to raise the threshold for tax increases.

"I'm not sure I could be more concerned, which is why I made the decision, over the objection of some people in this town, to pursue the referendum when it came to the Senate," California Republican Party Chairman Tom Del Beccaro said earlier this week.

Republicans stand to lose critical seats to Democrats under the Senate maps because of the state's shifting demographics, giving Democrats a better chance of reaching the critical two-thirds majority in the Senate. Democrats currently hold 25 of the 40 seats in the upper chamber and need just two more to hit the tax vote threshold.

The last time one party held a two-thirds majority in one house of the Legislature was after the 1976 election, when Democrats controlled the Assembly. In the Senate, it was 1965, when Democrats had a 27-13 majority, according to E. Dotson Wilson, the chief clerk in the Assembly.

The maps have put several GOP lawmakers on defense. State Sen. Sam Blakeslee, a Republican from San Luis Obispo, has indicated he is unlikely to run again because his Central Coast district now leans Democratic. Modesto Republican Sen. Tom Berryhill is an incumbent whose seat looks difficult to win. And Assemblyman Jeff Miller, R-Corona, appears to have a difficult fight after he announced he would seek a Senate seat.

If Democrats achieve that threshold, they would be able to approve tax increases without Republican support in one house of the Legislature. The independently drawn legislative districts and a new primary system were designed to favor more moderate candidates, theoretically leading to more bipartisanship in both houses of the Legislature.

Democratic political strategists cheered the Supreme Court's decision, but Senate President Pro Tem Darrell Steinberg, D-Sacramento, was restrained in his response. He said lawmakers have a lot of work ahead of them this year and that voters will be looking for results.

"Senate Democrats are pleased but aren't surprised that the Supreme Court wisely decided to uphold the clear will of California voters," he said in a statement. "Our focus has been and will continue to be on communicating directly to our constituents about what we are doing to strengthen the economy and our schools."

That caution is warranted, said Allan Hoffenblum, co-editor of the nonpartisan California Target Book, which studies legislative and congressional races.

While Democrats appear to have an advantage on paper, they still need to put up strong candidates in close districts, he said. In the last election cycle, moderate Republicans were able to win some seats that had been trending Democratic.

"Yes, the new lines give the Democrats a good opportunity to get a supermajority, but they have to win first," Hoffenblum said.

The 14-member citizens redistricting commission was created by voters in 2008 to take the authority for drawing legislative and state Board of Equalization districts away from the Legislature. In 2010, voters expanded the commission's authority to congressional districts.

While Republicans are challenging the congressional district lines in federal court, the GOP did not challenge the new Assembly maps because Democrats are less likely to gain two thirds of the 80 seats in that house.

Orange County Republican activist Julie Vandermost filed the lawsuit against the state Senate maps in October. She and GOP interests contend the commission did not meet the constitutional criteria for drawing the maps in a transparent way and in trying to keep communities together, violating the federal Voting Rights Act.

Supporters of the Republican referendum attempt have turned in petitions with 711,000 voter signatures. The secretary of state's office said it will determine by Feb. 24 whether the petitions have the roughly 505,000 valid signatures needed to qualify for the November ballot.

Republicans had asked the court to step in because their referendum was likely to qualify.

The Supreme Court first had to find whether it even had the authority to act because the GOP referendum has yet to qualify. The justices concluded that they did have jurisdiction and then upheld the maps drawn by the citizens commission. It found they were "clearly the most appropriate map to be used in the 2012 state Senate elections even if the proposed referendum qualifies for the ballot."

The Supreme Court justices had several options in considering the case: They could allow the use of the redistricting commission's maps, as they did; used the state Senate districts that have been in place since 2001; devised an alternate plan; or created temporary Senate districts out of pairs of Assembly districts drawn by the commission.

Peter Yao, the acting chairman of the citizens commission, called Friday's decision "a great victory for the people of California" and said it was important for the stability of the electoral process in the state.

"It is regrettable that these challenges, based on partisan self-interest, have cost precious taxpayer dollars to defend the work of the people's commission," said Yao, a Republican from Claremont.

Del Beccaro said the court ignored the text of the original ballot measure passed by voters, which said the Supreme Court should hold off on using the commission's boundaries if a referendum is likely to qualify.

"This is incredible, what they did," Del Beccaro said. "The Supreme Court agreed with us on the merits and then chose to ignore the constitution. They severely undermined the rule of law today."

Six of the seven Supreme Court justices were appointed by Republican governors. The exception is Justice Goodwin Liu, who was appointed last year by Gov. Jerry Brown.

All justices agreed the independent commission's maps should be used, rejecting the Republicans' contention that they should not be in use while they were being challenged. The court said it considered all possible maps to decide which one best complied with the law.

The court pointed out that unlike any of the other proposed maps, the commission's Senate map already has survived one legal challenge. In October, the state Supreme Court determined that a GOP petition seeking to throw out the commission's maps "lacked merit."

"We are aware of no basis upon which to reasonably question the legality of the commission's certified state Senate map," the court wrote in Friday's opinion. "This clearly distinguishes the commission-certified map from each of the alternatives proposed by petitioner."