PHOENIX - A federal court has agreed to give Republicans a chance to argue the state's 30 legislative districts should be redrawn for the 2014 election.

A special three-judge U.S. District Court panel rejected arguments by attorneys for the Independent Redistricting Commission that there is no legal basis for the lawsuit challenging the lines. District Judge Roslyn Silver, writing for the court, said the challengers are entitled to try to prove the commission purposely redrew district lines to improve Democratic opportunities.

But Silver cautioned that even if foes prove partisanship, it remains legally unclear whether that, by itself, is enough to invalidate the lines. She said that issue will have to be hashed out by the attorneys after a full-blown hearing.

The ruling could eventually undo the significant gains the Democrats made this election at the Capitol, picking up four seats in both the House and the Senate. If the judges order the lines redrawn, it could put Republicans into a better position two years from now.

Commission attorney Mary O'Grady on Monday downplayed the significance of the ruling. O'Grady said that while she had hoped to get the case dismissed, the decision does not mean the court ultimately will side with the challengers.

Central to the legal fight is a requirement all 30 districts have equal population. Using 2010 census figures, that should come out to about 213,000 in each of the districts. But the Independent Redistricting Commission, by its own admission, had districts ranging from 203,026 to 220,157.

What's worse, according to attorney David Cantelme, who represents the challengers, is many of those adjustments were made for political purposes.

In essence, he said, the commission "packed" Republican voters into some districts. That left underpopulated districts that gave Democrats a better chance of winning.

O'Grady told the court there were legitimate reasons for moving the original lines, including protecting minority voting strength and creating as many politically competitive districts as possible. Both are part of the 2000 constitutional amendment that took the decennial task of drawing lines away from the Legislature and gave it to the five-member commission.

Silver, however, said the opponents deserve their day in court.

"Regardless of what they can prove at trial, plaintiffs have sufficiently alleged that the commission added to some people's votes and diluted other people's votes based only on their expected party preference," she wrote.

Nonetheless, Silver wrote, "Under existing Supreme Court precedent, it is unclear whether pure partisanship is an arbitrary or discriminatory purpose and would therefore be an impermissible basis for population deviation."

Conversely, Silver said the court has to decide whether the voter-approved state law that created the commission even allows the panel to consider partisanship.

And she pointed out that, no matter what federal law allows, "the Arizona Constitution mandates equal population in the redistricting process."

The ruling clears a hurdle toward the bid by challengers for a full-blown hearing on the legal questions.

Given the time it takes to draw new lines, if that's what the court orders, Silver said she wants a hearing by the end of March.

There are two other legal challenges to the redistricting process in the works, both involving the lines for the state's nine congressional districts.

In one case also in federal court, Republican state lawmakers contend that the U.S. Constitution requires that congressional districts be drawn only by the Legislature, which invalidates the commission lines.

The other, in state court, charges the commission with misconduct in crafting the maps. A judge threw that case out but gave challengers a chance to refile.