Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

It’s the last open enrollment day for Obamacare, and the six-year-old health care reform law has never been more popular.

And it’s never been more doomed, although Republicans are still arguing over how and when to replace it — and whether to just tweak the Affordable Care Act and rename it, or completely repeal it and start over again from scratch.

This time last year, there was a huge last-ditch effort from the Obama administration to get people signed up in time to be covered for 2016 without paying an extra tax. This year, the Trump administration is virtually silent, with only the occasional post on social media and a broad cutback in pre-paid advertising.

Yet polls show the Affordable Care Act and its goals of getting health insurance and better health care to more people is growing in popularity as people come to understand they may be about to lose it.

The 2010 law got about 20 million more people covered by some form of health insurance. Fewer than 10 percent of Americans — about 28 million people — now have no health insurance.

A poll last week from the Associated Press and NORC Center for Public Affairs Research found that 56 percent of U.S. adults were "extremely" or "very" concerned that many will lose health insurance if there is a repeal.

And the poll found 53 percent want to keep the law in some form. That’s even more than an NBC News/Wall Street Journal poll taken the week before that found 45 percent believe the ACA is a good idea. That poll found that 50 percent of those surveyed have little to no confidence that Republican proposals to replace the law will make things better.

“The last four weeks have presented a lot of opportunity for confusion."

Private healthcare advocacy groups such as Enroll America say they’ve seen a surge of interest. “The last official numbers we received were from the former administration and that suggested that this year’s enrollment is outpacing last year’s,” said Enroll America’s Jennifer Sullivan.

But callers and people signing up in person are also asking a lot of questions.

Related: Obamacare's Never Been More Popular

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

“The last four weeks have presented a lot of opportunity for confusion,” Sullivan told NBC News.

People have heard about a vote in Congress making it possible to partly repeal the ACA, she said, and they are worried they’ll have the rug pulled out from under them if they carry private health insurance bought through the exchanges.

Leaked audio from a closed-door Republican meeting in Philadelphia obtained by the Washington Post last week revealed that GOP lawmakers are divided on how to proceed with their promise to "repeal and replace" the ACA. Republicans expressed concerns about messing with popular aspects of the law, such as provisions that stop insurance companies from turning away people who are too sick, capping coverage and letting adult children remain on their parents’ health insurance until age 26.

Related: What Can a New Trump Administration do to Obamacare

But the individual mandate — the requirement that people have insurance or pay a tax — is almost certain to go. And there is much discussion of changing the way Medicaid, the joint state-federal health insurance program for low-income and disabled people, is paid for and managed.

The uncertainty is making insurance companies, hospitals, doctors and health policy analysts nervous.

Mark Your Calendars: You have until January 31st to sign up for coverage.https://t.co/8Xdszf7WiY — CMSGov (@CMSGov) January 27, 2017

Only 15 percent of primary care doctors said they wanted the law repealed, according to a survey published last week in the New England Journal of Medicine, while 73 percent want to make improvements to it.

Insurance companies realize the mandate they insisted on back in 2009 is extremely unpopular, but are pressing for other ways to keep a good mix of healthy and sick customers, such as a requirement that people with pre-existing conditions pay premiums for a time, perhaps 18 months, before they can be eligible for full coverage.

Related: GOP ACA Replacement Would Give Power to States

The National Governors Association has written to members of Congress asking them not to shift more costs to states. The law calls for expansion of Medicaid and provides for the federal government to shoulder virtually the full cost. It also pays supplements to premiums and extra payments to insurance companies.

But some of the GOP plans being floated include Medicaid block grants — a flat payment to states to handle the program.

“This is a major, major change affecting millions of people in America,” said Diane Rowland of the Kaiser Family Foundation.

Related: Nearly 13 Million Sign Up for Obamacare

The Republican Medicaid proposals would seek to control Medicaid costs, which are unpredictable under the current system. But Rowland worries that a set budget would leave people out on the cold in an emergency.

“If there is an epidemic, or an outbreak … or if a new drug comes on the market that is very expensive …there would be no way to have those adjustments,” Rowland said. She pointed to the new hepatitis C drugs, which can cost hundreds of thousands of dollars but which can cure a previously incurable but expensive-to-treat condition, perhaps saving money over the long run.

“The Republican proposals would benefit higher-income people over lower-income people."

Women’s health may also suffer under the new proposals, said Kaiser’s Usha Ranji. She worried that insurance companies may be allowed to return to the old policies of considering pregnancy, or having had a cesarean section delivery, as pre-existing conditions. The ACA forbids that.

“Maternity care was not a required benefit,” Ranji said.

One proposal popular among Republicans would be allowing insurance companies to see policies in any state. Kaiser’s Larry Levitt said that could lead to people being tricked into buying truly bare-bones policies that offer little or no help when customers get sick or are injured.

People covered by pricey employer-based insurance would see little change, but the people the ACA aimed to help the most would suffer, he said.

“The Republican proposals would benefit higher-income people over lower-income people,” he said.