PHOENIX — Despite concerns of Democrats on the panel, the Arizona Independent Redistricting Commission on Thursday gave preliminary approval to a map that would give Republicans an edge in electing members of the legislature for the balance of the decade.

At least part of that is because they voted to create a legislative district in Pima County designed to help ensure the election of Republicans to the state House and Senate.

There was less controversy over a decision to give final approval to draft maps that also would give Republicans an edge in congressional races for the balance of the decade.

As crafted, Republicans would dominate three districts and Democrats in two. The margins are close enough in the other four to, at least theoretically, allow a candidate from either party to win a general election.

Even with Thursday’s vote, that is not the final word. Commissioners said they anticipate changes after the map goes out for public comment for the next 30 days.

One issue left hanging is how the map splits Tucson between two congressional districts following a complaint from Mayor Regina Romero. She said it makes no sense to have the University of Arizona and the nearby Fourth Avenue shopping and restaurant area separated from downtown.

“She makes a compelling argument,’’ said chairwoman Erika Neuberg.

But rather than wrestle with the issue Thursday, the commission chose to keep that line in place, at least for now, with the option of making changes before final adoption.

The real debate Thursday came over how to draw the lines for the state’s 30 legislative districts, with the Republican and Democratic commissioners on opposite sides of the key battle of how to divide Southern Arizona.

Neuberg sided with the two Republicans in saying she prefers a plan that allows what she called “right-of-center’’ residents in the Tucson area to have some chance to elect someone who shares their political philosophy.

But in doing so, the commission rejected a proposal by Shereen Lerner, one of the two Democrats, to create a district with a population split nearly even between the two major parties. She argued that was fair.

“It provides Republicans with a good opportunity to have their voices heard,’’ Lerner said.

The maps create two legislative districts in which a study of how residents of the area voted in prior elections make them an effective tossup. Another four districts are considered within the possible competitive range, with two leaning Republican and two leaning Democrat.

There would have been three tossup districts, with 12 where the margins favor Republicans and 11 with decidedly Democratic edges.

That was before the panel crafted LD 17 — the one in the suburban Tucson area — to give an edge to the GOP. Hence, the draft maps appear to create 13 Republican-leaning districts against 11 with a Democratic edge.

The commission did that by crafting a district that starts at Marana, runs through Oro Valley and around Tucson’s northern and eastern edge to take in Tanque Verde and Vail.

Lerner proposed instead to include Casas Adobes in the district, saying that makes more sense.

“They share a lot of services in that area,’’ Lerner said. It also would have kept most of the Amphitheater and Catalina Foothills school districts in the same political area.

That did not sit well with commissioner David Mehl.

He agreed to back off — but just a bit — from the plan he had previously advanced on behalf of the Southern Arizona Leadership Council, of which he is a founding member.

His earlier plan included Oracle, Mammoth and San Manuel in LD 17. They now are in LD 7, which stretches north through Florence into the eastern edge of Apache Junction and up to Payson and Snowflake.

And the Flowing Wells area was moved from LD 17 into LD 20, which runs through downtown Tucson.

But Mehl insisted that with four strongly Democratic districts in Pima County there needs to be at least one with a Republican edge.

“Having a district that leans Republican I don’t think is a bad thing,’’ he said.

Mehl argued this plan creates a “suburban ring around Tucson.’’

He also said it also reflects that Tanque Verde residents are in many ways like those living in the city’s northern suburbs. Mehl said they have no interest in being aligned with people living in Tucson, saying they have repeatedly rejected annexation.

He got the support of Douglas York, the other Republican on the panel, who said it makes sense to have Vail and Tanque Verde linked to the northwest section of the county.

“The east side of Pima County is a lot like the Marana area,’’ he said.

Lerner, a Democrat, said her GOP colleagues were ignoring one requirement they have for drawing lines: creating compact districts.

“Casas Adobes, it’s right there,’’ she said. “We’re going around it for political reasons.’’

What tilted the debate in Mehl’s favor was the support of Neuberg, the lone independent on the commission.

“I am focused on ensuring some accountability in the Tucson area for right-of-center folks, a community of interest, to not be neglected,’’ she said.

The result is that LD 17 has a Republican edge, versus what would have been a nearly even political split under Lerner’s plan.

That question of what the area around Tucson should look like legislatively spilled over into a separate debate over the political future of southern Cochise County, specifically Bisbee and Douglas.

“These are border communities that have high Latino populations,’’ said Lerner.

But the map given preliminary approval Thursday puts them into heavily Republican LD 19 which includes not just the rest of Cochise County but goes as far west as Green Valley and stretches to Safford, Clifton and Morenci.

Lerner wants those areas linked to LD 21, a heavily Democratic district that runs from Nogales into southeast Tucson.

The problem is that the law requires each of the 30 legislative districts to have about equal number of residents. Removing the populations of Bisbee and Douglas would leave LD 19 short.

Lerner said there’s a simple way to solve that: Add Vail and Tanque to LD 19. But that drew objections from Mehl, who said he doesn’t want to give up those Republican voters from LD 17