The new year may be looking brighter for Obamacare insurers after a surge in sign-ups for 2017 coverage, with experts noting that Republican talk of repealing the healthcare law is sparking some of the increase.

About 6.4 million people had signed up for Obamacare as of Dec. 19, 400,000 more sign-ups than during the same period last year. The surge of sign-ups comes as the Republican-controlled Congress has put repealing the healthcare law at the top of its agenda when it returns next month.

The threat of repeal hasn't done much to halt sign-ups for the controversial healthcare marketplaces, which had a record-breaking 670,000 sign-ups on one day this month as the deadline to get coverage by Jan. 1 approached.

Customer renewals have represented the bulk of the surge of sign-ups, with 4.31 million renewing their coverage for 2017. New customers accounted for 2.05 million sign-ups.

"New enrollment is down a bit from last year but is generally holding steady," said Larry Levitt, senior vice president for the nonpartisan Kaiser Family Foundation.

The Obama administration set an ambitious goal for sign-ups for 2017, hoping for about 13.8 million to pick plans and about 11 million to pay their premiums. The sign-ups would be about 1 million more than the last open enrollment.

Levitt said many factors could be causing the jump in renewing customers, including worries about Republican promises to repeal the law and people shopping around for lower-cost plans in the face of major premium hikes, and some insurers exiting the market, prompting the customers left behind to find new plans on the marketplace.

One financial analyst said that the promises to repeal the law are having an impact on sign-ups.

"We have heard anecdotally that previously uninsured that are younger and healthier are enrolling on the marketplaces given the fear of future repeal," said Ana Gupte, managing director of health services for financial analyst firm Leerink, in an article in Forbes.

More sign-ups could go a long way to stabilize the marketplace, Levitt said.

"If left alone, the marketplaces seem stable for now," he said.

That would be a drastic departure from 2016, as mounting financial losses caused major insurers UnitedHealth and Aetna to depart from most of the states they offer Obamacare plans in.

The insurers that decided to remain raised rates to compensate for higher claims costs from a sicker-than-expected enrollee population. The Obama administration said in the fall that the average plan's premium would rise by 25 percent, and some states approved increases of 50 to 60 percent.

The administration has hoped to expand outreach to younger and healthier people to get them to sign up, hoping that an infusion of younger people could improve the marketplaces' risk pools.

The signup surge comes in the midst of a new report from Standard & Poor's that predicts that Obamacare insurers will fare better in 2017 than they did in 2016.

The report said the major price hikes are likely to be a "one-time pricing correction," according to a CNBC report. However, the research firm said it was hard to predict the healthcare market in 2018 and beyond.

That is due to Congress' intent to repeal the healthcare law, which Levitt cited as a factor in the spike in renewals.

Congress hasn't coalesced around a replacement plan for Obamacare. The House can repeal the law easily and avoid a filibuster in the Senate by using a procedure called reconciliation. After repeal, House and Senate aides have said that work will start quickly on drafting a replacement. There likely will be a transition period of up to four years to approve and install a replacement, GOP aides have said.

"While the Affordable Care Act didn't produce an insurance death spiral as critics warned, repealing the law could do just that if it's not done carefully," Levitt said.

He added that the full picture of enrollment isn't known yet, since open enrollment ends Jan. 31.

Levitt added that experts don't know what the numbers are for "enrollment among people in the individual market outside the marketplaces, which may be vulnerable given the big premium increases for 2017."

The premium increases affected all plans sold on the individual market, which is for people who don't get insurance through their job. Obamacare's marketplaces comprise a majority of the individual market, but there are plans that are sold off those exchanges.

Anyone who buys plans off the exchange isn't able to get the law's tax credits to pay down the cost of health insurance. Low-income Obamacare customers are likely to be immune from big premium increases as tax credits rise with any hike in premiums.