Former President Donald J. Trump said in a video posted to his social media site on Thursday that he was “not running to terminate” the Affordable Care Act, his latest effort to push back on attacks from the Biden campaign and other Democrats over his calls to replace the legislation known as Obamacare.

In a statement similar to one he posted in March, Mr. Trump said the health care law was “much too expensive” and “not very good,” and added, without providing specifics, that he would improve it if re-elected. Those statements came after he drew criticism for having again suggested, in the last few months, that he would repeal the law, which has become more popular since its enactment 14 years ago.

“We’re going to make the A.C.A. much better than it is right now and much less expensive for you,” Mr. Trump said in the video.

On Tuesday in Washington, President Biden accused Republicans of aiming to “terminate the Affordable Care Act” at an event for caregivers, where he promoted expanding paid medical leave, child care subsidies and early education.

Also this week, Mr. Trump shifted his stance on abortion. For months, he gave mixed signals on his position, before saying in a statement on Monday that abortion policies should be left to the states.

The former president’s pledge to repeal the health care legislation dates to his first presidential campaign in 2016, when many Republicans were issuing calls to “repeal and replace” the law. But Mr. Trump did not offer a substantive proposal to replace it, and efforts backed by his administration to repeal parts of the law were rejected by Congress and the Supreme Court.

For months, the Biden campaign has attacked Mr. Trump on the issue, after he described the Affordable Care Act last November as “out of control” and said he was “seriously looking at alternatives” to it. The Biden campaign quickly amplified those remarks, and Mr. Biden offered his own criticism last November, saying, “My predecessor once again called for cuts that could rip away health insurance for tens of millions of Americans.”

Mr. Trump’s response on Thursday echoed a post he made on social media in March, in which he claimed, in all caps and with a misspelling of Mr. Biden’s name, that “crooked Joe Buden disinformates and misinformates all the time.” That post came days after the Biden campaign released a digital ad in battleground states that featured audio of Mr. Trump criticizing the Affordable Care Act and claiming that Mr. Trump “tried to rip away our health care” as president.

More than 45 million people are enrolled in Affordable Care Act-related coverage, according to a recent report from the Department of Health and Human Services. A record number of people — more than 20 million — have signed up for plans on the act’s marketplaces this year, according to the Biden administration. Mr. Biden proclaimed that the figure proved that the legislation “is more popular than ever.”