WASHINGTON (AP) — Americans can start signing up Friday for health care coverage offered through the Affordable Care Act marketplace for 2025, days before a presidential election that could threaten eligibility and raise costs for millions of those in the program.

The future of 'Obamacare' has emerged as a key issue in the closing days of the presidential campaign, with a top GOP leader promising this week to overhaul the program should Republican Donald Trump win the presidency.

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

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

"Americans can actually look at health care not as a privilege, but now as a right," said Becerra, who is going to the political swing state of Arizona on Friday to mark the beginning of open enrollment. "I don't think anyone wants to have a right yanked away from them to have affordable health insurance coverage."

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

On Monday, a key Trump ally, House Speaker Mike Johnson, declared "No Obamacare" during a Pennsylvania campaign stop, promising "massive reform" under a Trump presidency with a Republican-controlled Congress.

"Health care reform's 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 that he had no plans to completely repeal the landmark legislation, but the comments still underscored how Johnson is working closely with Trump to potentially remake the federal government and its social programs if Republicans sweep into power.

Trump has only said that he has "concepts of a plan" to change the Affordable Care Act, which he was unable to successfully scuttle altogether during his previous term in office. In recent months, Republicans have raised concerns about spending and fraud.

Enrollment under the Biden administration surged, after years of declines during the Trump administration, dropping to a low of 11 million on Trump's watch. But taxpayers have had to pour billions of dollars more into the Affordable Care Act to achieve that increase. That money has helped pay for subsidies to cut premiums by nearly half for many of those enrolled in the program.

Congress would need to pass a new law to continue offering those subsidies, which could cost as much as $335 billion over the next decade. Some Republicans have already balked at that figure.

Democrats have cautioned voters that Republicans will try to revive an attempt to kill the Affordable Care Act altogether, with the warnings growing more dire as Election Day nears.

In Wisconsin, Vice President Kamala Harris issued the warning again, telling reporters that Johnson's remarks prove Republicans plan to gut the health care coverage. " It has been a 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 that agenda from his 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 pull back on. For example, the administration poured more money into hiring health insurance navigators who help enroll people into coverage.

The White House also implemented a new rule that expands eligibility for the program to immigrants who were brought to the U.S. as children. About 100,000 of those immigrants, also called "Dreamers" are expected to sign up for the coverage for the first time during enrollment this year.

Republican attorneys general in 15 states have sued to block their enrollment.

Open enrollment for the coverage ends on Jan. 15, but to have coverage in the new year you must enroll by Dec. 15.

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