WASHINGTON — Insurers are fretting about the Senate Republican bill to replace the Affordable Care Act because it would repeal the individual mandate but — unlike the House bill — not provide an incentive for people to retain coverage.

Abolishing the health care law’s mandate with no penalty for coverage lapses would destabilize the individual insurance market by leading healthy people to drop their coverage, leaving insurance companies with sicker, more costly plan members in their enrollee risk pool.

As a result, premiums would increase as much as 20 percent, according to government estimates, making coverage unaffordable. And people with medical problems could game the system by enrolling only when they get sick.

“Particularly, if there’s something like, you know, you have to have knee surgery in April. And then sign up for coverage, get the surgery and then drop it,” said Katie Allen, executive director of the Council for Affordable Health Coverage. To avoid that scenario, the House Republican repeal legislation imposes a 30 percent surcharge on people who let their individual insurance lapse.

But Senate Republicans didn’t include the surcharge in their legislation because the provision conflicts with the Senate’s “Byrd Rule,” a parliamentary rule that governs what can be included in a budget reconciliation bill.

“While we recognize that the absence of these provisions may be due to concerns related to the Byrd Rule, it will be incumbent on Congress to offer a fix to this omission— either by revising the draft bill or passing separate, bipartisan legislation,” said a statement from Joel White, president of the council, a coalition advocacy group that includes insurers and pro-business groups.

The mandate, long the most unpopular provision of the Affordable Care Act, requires most people to have insurance coverage or pay a penalty.

Because the health law guarantees access to individual coverage for people with pre-existing conditions and bars insurers from charging sick people more for coverage, the individual mandate helps ensure that a mix of healthy and sick people are covered. That helps keep premium costs in check.

In a May letter to Senate Finance Committee Chairman Orrin Hatch, R-Utah, America’s Health Insurance Plans, the lobbying association for health insurers, said any legislation that repeals the mandate “must include alternatives to incentivize continuous coverage.”

Repealing the mandate without the incentives would cause premiums to increase 20 percent, the non-partisan Congressional Budget Office has estimated.

But the Congressional Budget Office, known as the CBO, also found that the 30 percent surcharge is no magic bullet either, said Jeanne Lambrew, a senior fellow at liberal Century Foundation.

“The CBO estimated that the surcharge would do more harm to the risk pool because people who are sick and have high medical costs would be willing to pay the surcharge to get the needed coverage,” Lambrew said in a conference call with reporters on Friday.

Andy Slavitt, former acting administrator of the federal Centers for Medicare and Medicaid Services in the Obama administration, said Republicans “don’t have a good answer.”

“Maybe they will, over the next week, try to put something back in,” Slavitt said during the press call. “But I think we all ought to be very sober to the fact that there’s nothing really that can be done to fix the situation that they’re in right now.”

Not so, said Tara O’Neill Hayes, deputy director of health care policy at the conservative American Action Forum.

In a blog post, Hayes outlines several options that Republican Senate aides could consider in place of the 30 percent surcharge. Those options include waiting periods to access insurance benefits for people who have let their coverage lapse.

“The individual would be required to begin paying monthly premiums, but coverage would not go into effect until the waiting period is over,” Hayes wrote. Insurers imposed similar waiting periods for individual coverage before the Affordable Care Act became law.

Another possibility would be to limit plan choices for people who’ve had coverage lapses, allowing only catastrophic coverage or plans that cover a lower share of medical costs.

“Under this option, it would likely be necessary to couple such a provision with an inability for the individual to simultaneously receive a tax credit, as the amount of the tax credit would likely cover the entire cost of such a plan, mitigating any incentive to maintain coverage and making such a policy less effective,” Hayes added.

Another incentive to maintain coverage would be to restrict eligibility for tax credits among people who let their coverage lapse.

Allen said the council is also amenable to people losing their tax credit altogether if they don’t maintain continuous coverage.

Republican Senate staffers are expected to work with the Senate parliamentarian on a policy solution this week, Allen said.