PHOENIX — Organizers of a bid to block Medicaid expansion in Arizona conceded they may not have the necessary petition signatures to force the issue to the ballot.

“We’re over 70,000,” former state Sen. Frank Antenori said. Ron Gould, another former state senator involved in the effort, said there are last-minute efforts to gather up the petitions that have not yet been turned in.

“So it’s going to be close,” Antenori said.

But Antenori said it’s not simply a question of getting the 86,405 signatures legally required to stall enactment of the plan approved by the Legislature until voters have a say at the next election. He acknowledged it’s not unusual for a third or more of signatures on petitions to be declared invalid for various reasons, such as a person not registered to vote.

“It’s probably going to be tough to ward off a decent challenge,” Antenori said.

Meanwhile, a separate referendum challenging changes in election laws approved by the Republican-controlled Legislature appears on track to qualify. That group has scheduled a news conference for Wednesday afternoon to turn in its signatures.

“We know we’re over 100,000,” said Julie Erfle, who is chairing that effort. “That’s all we’re saying right now.”

But Erfle said she is “very, very confident we’ll be on the ballot.”

Money appears to be a key difference.

The bid to force a public vote on the election law changes was fueled by donations of more than $300,000, enabling it to gather the signatures it needed.

By contrast, the effort to block Medicaid expansion was premised on using a network of volunteers drawn from the ranks of the Republican Party to get the signatures. Late last month, the organization got its first and only large donation: $20,000 from the Tea Party Patriots.

At the same time, backers of Medicaid expansion raised more than $350,000 from business interests and hospitals to block the referendum. Their tactics included hiring people to circulate their own petitions promoting a larger Medicaid program even though those petitions have no legal force or effect.

Antenori said that maneuver left his group with few people available to circulate referendum petitions even after he got some money.

The fight is over a measure pushed through the Legislature by Gov. Jan Brewer to impose what amounts to a tax on hospitals to bring 300,000 or more into the Arizona Health Care Cost Containment System.

Arizona currently provides care for most individuals below the federal poverty level, about $19,530 a year for a family of three. That amounts to about 1.3 million residents, with the federal government picking up about two-thirds of the cost.

Several years ago, though, the state stopped enrolling single adults in the program even if they were otherwise qualified.

Since then, Congress approved the Affordable Care Act, providing generous federal subsidies to states that expand coverage to 138 percent of the poverty level.

The legislation both expands eligibility and restores coverage for the single adults who have been shut out of the program using those hospital taxes to cover the state matching share. The hospitals went along after being shown figures by the Brewer administration that they would make more in AHCCCS payments than the extra tax would cost them because they would have fewer patients unable to pay their bills.

Foes contend the federal funds are unsustainable, saying the state will be stuck with the entire burden when Washington defaults.

The measure does have a “circuit breaker” scaling back the program if federal funding drops below 80 percent of the cost. But opponents say that is insufficient to protect against political pressure to maintain coverage.

Others opposed to the law have different concerns, including some who have labeled the whole federal program “socialized medicine.”

Even if the referendum fails, another hurdle awaits.

A provision of the Arizona Constitution requires that any increase in taxes or revenues to the state must be approved by at least a two-thirds vote of both the House and Senate.

The expansion plan did not get that margin. But supporters and Brewer contend the levy on hospitals is an “assessment,” not a tax — a point certain to be challenged in court.

The expanded program is supposed to kick in on Jan. 1.