About a month before state Senate candidates must file for the June primary election, the California Supreme Court wrestled Tuesday with which district boundaries should be used if a pending referendum qualifies for the ballot.

A decision by the high court is expected this month on the issue, created by the filing of more than 711,000 signatures by a Republican-backed group seeking to overturn new Senate maps drawn by the state's Citizens Redistricting Commission.

In a 75-minute hearing, justices focused on technical issues, including whether they can rule only if the referendum is "likely to qualify" and what standard should be used to measure that.

Justices also questioned attorneys for the redistricting commission, the secretary of state's office and the Republican-backed group that led the drive – Fairness and Accountability in Redistricting – about options if the court decides to intervene.

Twenty Senate seats are up for grabs this year. The issue carries high stakes politically, because many analysts have said the new districts could give Democrats a strong chance of gaining two additional seats in the Senate, enough to gain the two-thirds supermajority needed to raise taxes or fees.

County elections officials are determining whether FAIR's petitions contain the 504,760 valid voter signatures to qualify for the ballot. Counting is not expected to be completed until late February.

Judging from justices' questions, a key issue is whether the state Supreme Court could order the newly drawn Senate districts to be used for the 2012 elections even if the referendum qualifies.

Attorney Charles Bell, representing FAIR, told the justices that using the contested maps would not be fair to voters who have exercised their legal right to challenge them.

Attorney James Brosnahan, representing the redistricting commission, said the new Senate maps should be used this year, even if the referendum qualifies for the ballot, because they best comply with federal voting rights law and voter-approved criteria.

Other possibilities include appointing a special master to draw new maps; using Senate maps that were in place from 2001 to 2011; or ensuring that two of the redistricting commission's new Assembly districts are combined – a process called "nesting" – to create each new Senate district.

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