WASHINGTON – The Americans can start signature Friday for health care coverage offered through the Affordable Care Act’s Marketplace for 2025, days before a presidential election that could threaten eligibility and increase costs for millions of people in the program.

The future of Obamacare has emerged as a key issue in the final days of the presidential campaign, with a top Republican Party leader vowing this week to overhaul the program if Republican Donald Trump wins the presidency.

Billions of dollars in tax credits are also at stake. Created during the COVID-19 pandemic, the money has expanded eligibility for millions of Americans, made health insurance coverage more affordable for many and increased registrations considerably.

Nearly all of the 21 million people in the program have benefited from the grants, which expire next year, U.S. Health and Human Services Secretary Xavier Becerra said in an interview with The Associated Press.

“Americans can actually view health care not as a privilege, but now as a right,” said Becerra, who will travel to the politically swing state of Arizona on Friday to mark the start of open enrollment. “I don’t think anyone wants to have their right to have affordable health insurance coverage taken away.”

But significant changes to the agenda are almost certain if Trump wins the White House and Republicans take control of Congress in Tuesday’s contentious elections. They are threatening to roll back Obamacare, a signature achievement of former President Barack Obama, a Democrat.

On Monday, a key Trump ally, House Speaker Mike Johnsondeclared “No Obamacare” during a campaign stop in Pennsylvania, promising “massive reform” under a Trump presidency with a Republican-controlled Congress.

“Health care reform is going to be a big part of the agenda,” Johnson said in a video first obtained by NBC News.

Johnson’s office later issued a statement saying he did not intend to completely repeal this landmark legislation, but the comments nevertheless highlighted how closely Johnson was working with Trump to potentially overhaul the federal government and its social programs if the Republicans came to power.

Trump said only that he had “an idea for a plan” to change the Affordable Care Act, which he failed to defeat during his previous term. In recent months, Republicans have raised concerns about spending and fraud.

Enrollment under the Biden administration increased, following years of decline under the Trump administration, falling to a low of 11 million under Trump’s leadership. But taxpayers had to invest billions more in the Affordable Care Act to achieve that increase. This money helped fund subsidies that cut premiums by almost half for many people enrolled in the program.

Congress should pass new legislation to continue offering these grants, which could cost up to $335 billion over the next decade. Some Republicans have already balked at this figure.

Democrats have warned voters that Republicans will try to revive their bid to completely gut the Affordable Care Act, with the warnings becoming increasingly dire as Election Day approaches.

In Wisconsin, Vice President Kamala Harris issued that warning again, telling reporters that Johnson’s remarks proved that Republicans were planning to gut health care coverage. “This has been part of Donald Trump’s agenda for a very long time,” Harris said. “He has made dozens of attempts to get rid of the Affordable Care Act. And now we have further validation of this program from its supporter, the Speaker of the House.

The Biden administration has expanded the program in other ways that Harris is likely to continue, but Trump is sure to withdraw. For example, the administration has invested more money in hiring health insurance navigators who help people enroll in coverage.

The White House also implemented a new ruler which expands eligibility for the program to immigrants who were brought to the United States as children. About 100,000 of these immigrants, also known as “Dreamers,” are expected to sign up for coverage for the first time when they sign up this year.

Republican attorneys general from 15 states have continued to block their registration.

Open enrollment for coverage ends January 15, but to receive coverage for the new year, you must enroll by December 15.

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AP journalists Lisa Mascaro and Stephen Groves contributed to this story.