Very little that President-elect Donald Trump said during his16-month campaign for president about health care was clear orconsistent. But there was one vow he stuck by at all times: Hewould allow insurers to sell health plans across statelines.

“Modify existing law that inhibits the sale of health insuranceacross state lines,” states his website’s health care policysection. “As long as the plan purchased complies with staterequirements, any vendor ought to be able to offer insurance in anystate. By allowing full competition in this market, insurance costswill go down and consumer satisfaction will go up.”

Trump’s belief that making the health plan market national will result inlower costs and better products for consumers is not uncommon inconservative circles. It’s been a rallying cry for free marketenthusiasts for years.

Many health care policy experts, however, are rolling their eyesin response.

For starters, many states have already passed laws to allowout-of-state insurers to enter their market. But few insurers optto do so because of the hard work it will take to build up anetwork of providers in the new state.

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“The barriers to entry are not truly regulatory, they arefinancial and they are network,” Sabrina Corlette, the director ofthe Georgetown University Health Policy Institute, said in aNew York Times interview last year.

A study that Corlette conducted in 2012 found that states thathad passed laws aimed at welcoming out-of-state insurers hadproduced zero interest from the insurance industry. Not one insurerresponded to such laws by selling plans in a new state.

The federal government could more aggressively try to end thefragmentation of the insurance market by barring states fromimposing many of their own regulations on health plans.

But consumer rights advocates say such a change would leavepolicyholders in many states in the lurch. Health insurers wouldrespond by setting up headquarters in the lowest-regulation state,much in the way that credit card companies moved to Delaware in thewake of a financial deregulation law in the 1970’s.

If insurers all set up shop in a few low-regulation states, itis likely they won’t go to great lengths to develop robust providernetworks for customers in certain parts of the country,particularly sparsely populated rural areas. With a national marketin mind, it might not be worth it for a Texas-based insurer to tryto provide plans to customers in Wyoming.