As merger-mania sweeps the health-insurance industry, the plans involved are banking on regulators at the Department of Justice  to clear their mergers by the end of next year.

But doctors, hospitals, and other critics are hoping to drag out scrutiny of the deals beyond 2016 so federal antitrust regulators take a closer look, potentially derailing them based on opponent worries about unfair market clout if the so-called "Big 5" insurers dwindle to just three national carriers.

Anthem (NYSE: ANTM) is poised to become the nation's largest health insurer with its $54 billion acquisition of Cigna (NYSE: CI), while Aetna (NYSE: AET) has agreed to buy Humana (NYSE: HUM) for $37 billion to become the nation's third largest insurer. UnitedHealth Group (NYSE: UNH), currently the nation's largest health plan, would fall to No. 2.

The uncertainty that will hang over this unprecedented consolidation is something for investors to watch because it could interrupt a historic bull run for these health insurer stocks just as they're gaining millions of new customers under the Affordable Care Act.

Both Anthem and Aetna have told analysts and investors that their acquisitions should close in the second half of 2016 -- even as organizations such as the American Medical Association and allies including the American Hospital Association marshal their forces to derail the deals.

Justice Department on "no clock"
"The insurers are being fairly optimistic, and I think that is a pretty ambitious schedule," says David Balto, a former policy director at the Federal Trade Commission who is now a private antitrust attorney in Washington. "There's no clock on the Department of Justice and a merger is forever, so they will take as long as necessary to address this."

Balto is among a growing chorus of critics of the mergers. In part, opponents such as the American Medical Association, the American Hospital Association, and others say the deals will reduce competition and consumer choice.

The reduction of five national carriers to just three could also give health plans unprecedented leverage to squeeze providers of medical care, particularly with national accounts and employers. Already, doctors and hospitals say insurers are narrowing their networks and excluding providers based on costs, and fewer health plans will mean more of this practice.

Doctors, hospitals fear being squeezed
"We believe that consolidation of large commercial insurers is the root of higher prices for consumers," says Melinda Hatton, general counsel at the American Hospital Association. "We would expect the antitrust division to ensure that none of these new consolidations would add to that problem."

UnitedHealth Group's 2008 acquisition of Nevada-based Sierra Health Services took 11 months to review in the face of an outpouring of criticism from providers, but it was much smaller than the mergers currently being considered. An AMA study of the Sierra deal said health-insurance premiums increased by "almost 14% relative to a control group" after that merger.

"To give commercial health insurers virtually unlimited power to exert control over an issue as significant and sensitive as patient healthcare is bad for patients and not good for the nation's healthcare system," AMA president Dr. Steven Stack said in a statement. "The U.S. Department of Justice has recognized that patient interests can be harmed when a big insurer has a stranglehold on a local market. Antitrust laws that prohibit harmful mergers must be enforced and anticompetitive conduct by insurers must be stopped."

Balto said opponents of the deal should "marshal their forces together and lobby to prevent the deal."

"They should follow the playbook of the (opponents) of the Comcast-Time Warner deal," Balto said of the giant cable deal that collapsed this spring. "They have to work together to derail this deal." As a refresher, Comcast and Time Warner ended their deal after DOJ attorneys said they planned to block the deal. Six U.S. Senators also urged the government to block the deal, citing an unfair monopoloy that would hurt consumers.

Medicare competition questioned
Providers were given some help this week in their battle thanks to a new report from the Commonwealth Fund that showed there is a lack of competition among Medicare Advantage plans, which are owned by the Big 5 insurers and contract with the federal government to provide benefits to seniors.

The report said there is "little or no competition" in Medicare Advantage insurance markets in 97% of U.S. counties. The Commonwealth Fund said its researchers used a "standard measure of market competition" from the U.S. Department of Justice and the Federal Trade Commission.

Balto said the Commonwealth report "shows that seniors pay more for Medicare advantage when there is less competition -- a lot more. These mergers will weaken an already competitively fragile situation."

But the insurance companies say the data in the Commonwealth Fund is four years old and doesn't look at the entire market of Medicare beneficiaries.

"The Medicare space is highly competitive," said Aetna spokeswoman Cynthia Michener. "The largest competitor is traditional fee-for-service Medicare, which remains the choice of more than two-thirds of beneficiaries. A combined Aetna-Humana would serve only 8% of the current 54 million Medicare beneficiaries, with the remainder being served by the government and more than 140 private insurers."

Given the unprecedented consolidation and clout the insurers would have nationally, I'd say there is a serious risk that the mergers could be blocked and ultimately slow the momentum of a hot sector of the healthcare market. Insurers say they are confident the deals will go through but haven't ruled out potential business divestitures. They aren't clear on what could be divested, however, so investors need to brace for potential unknowns that could put a cloud over the stocks during this period of regulatory scrutiny. 

Bruce Japsen has no position in any stocks mentioned. The Motley Fool recommends Anthem and UnitedHealth Group. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.