PHOENIX — The Arizona Supreme Court on Friday upheld a law that its proponents admitted is designed to help elect Republicans by keeping Libertarians from siphoning away votes.

In a unanimous ruling, the justices acknowledged the new law increased the number of signatures a Libertarian is required to get to run for statewide office by a factor of 25. By contrast, the 2015 statute had little effect on Republican and Democrat candidates.

But Chief Justice Scott Bales said the increased burden is legally irrelevant. He said the Republican-controlled Legislature was entitled to decide that all candidates from all parties have to show the same level of support to qualify for a place on the ballot.

Friday’s ruling comes in a case involving Libertarian Frank Tamburri. It dashes his bid to challenge Republican John McCain in November for the U.S. Senate.

But it may not be the last word on the validity of the law.

The Arizona Libertarian Party has a separate lawsuit pending in federal court challenging the same signature requirements. Oliver Hall, the attorney in that case, wants a judge to rule that the 2015 law imposes “unconstitutionally severe and unequal burdens on the Libertarians alone.”

On paper, the law treats all parties the same: They need the signatures of one-fourth of 1 percent of eligible voters. That’s down from one-half of 1 percent.

But lawmakers added a twist by redefining who are “eligible voters” who can sign the petition.

The original law included only members of that person’s party. Now the percentage is calculated based on not just party registration but also anyone registered as a political independent.

With the decrease in the percentage requirement by half, that’s no real change for Democrats or Republicans.

But the situation is different for Libertarians: While under the old law they needed just 133 signatures for a statewide candidate to qualify for the ballot; the new law raises that to 3,034. And there are similar increases in congressional and legislative races.

The record shows there was a political motive behind the change. During the debate over the 2015 measure, Sen. Steve Yarbrough, R-Chandler, conceded as much.

“What we have is that people who have historically been third-party candidates (have) been able to get on the ballot with an incredibly small number of signatures,” he complained.

Rep. J.D. Mesnard, R-Chandler, who sponsored HB 2608, said it was designed to stop the kind of games that can be played to siphon votes.

“My mom in CD 9 (in Tucson and southeast Arizona) was getting mail from the Democratic Party backing the Libertarian candidate,” he said. Mesnard said allowing a minor party candidate to qualify for equal ballot status with Republicans and Democrats without getting the same number of signatures “makes a mockery of the system and also opens it up for manipulation.”

He contends that’s already happened.

When the issue was first debated, Mesnard said that one or two of the 2012 congressional races did not go “in the direction I would have liked to have seen them go.” More to the point, Mesnard contended that the results would have been different had the higher signature requirement been in place and, presumably, Libertarian candidates would not have been on the ballot.

In CD 1, which runs from Flagstaff and the Navajo Nation to the edge of Tucson, Republican Jonathan Paton fell short in his bid to oust incumbent Democrat Ann Kirkpatrick.

Paton garnered 113,594 votes against 122,774 for Kirkpatrick. But Libertarian Kim Allen picked up 15,227 votes — votes Mesnard contended likely would have gone to Paton.

Similarly, in the newly created CD 9, which encompasses parts of Phoenix and Tempe, Democrat Kyrsten Sinema bested Republican Vernon Parker by 10,251 votes, with Libertarian Powell Gammill tallying 16,620.

And to drive the point home to his GOP colleagues who were voting on the measure, he pointed out that the new requirements also cover legislative races. Mesnard told them that if they didn’t support the change, they could be personally affected in their next race.

“I can’t believe we wouldn’t see the benefit of this,” Mesnard told fellow Republicans.

None of the political debate was discussed by Bales in the opinion he wrote for the court. Instead he focused purely on the numerical change.

“HB 2608, like any restriction on a candidate’s ballot access, imposes some burden on Tamburri’s political opportunities,” the chief justice wrote. And it might even “limit the field.”

But Bales said that a “reasonably diligent” minor party candidate could still gain access to the ballot, saying that getting signatures from one-quarter of 1 percent of all eligible voters — Libertarians and independents — is not an unreasonable burden. He said the Legislature has a “legitimate interest in ensuring candidates who appear on the general election ballot have some significant modicum of support.”