Former President Barack Obama took to Twitter Wednesday morning to encourage people to shop for Affordable Care Act health insurance.

Obama's rare appeal comes as his signature health care law is under attack by his successor, President Trump, and Republicans in Congress.

Obama's tweet to his more than 95 million followers includes a short video, set to jaunty music, where the former president urges people to log on to the federal insurance exchange, HealthCare.gov, and sign up for coverage for next year.

"It's November 1, which means today is the first day to get covered for 2018," Obama says. It's not clear where he's standing, but the ocean is in the background.

While the point of the video, which is on Obama's Facebook page, too, is clearly to gin up business for HealthCare.gov, Obama takes the opportunity to defend his signature achievement. He mentions two of the law's most popular provisions, both of which have been threatened by Republicans.

"Thanks to the Affordable Care Act, insurance companies can no longer charge you more just for being a woman, or having a preexisting condition — and that's a good thing," he says.

Republicans in Congress have tried over and over this year to repeal the Affordable Care Act and replace it with an alternative plan. Those efforts have failed in large part because most of the alternative ideas would end with millions fewer people having health coverage.

Still, President Trump has repeatedly said the Affordable Care Act markets are collapsing. Critics say Trump's actions have weakened those markets because they've injected uncertainty into the system, leading companies to either raise premiums or pull out of the system altogether.

Average premiums for 2018 will be higher than in 2017, according to the Department of Health and Human Services. But those higher premiums are more than offset by higher government subsidies, for the 85 percent of consumers who buy insurance on the ACA exchanges and qualify for financial assistance.

An HHS analysis released this week shows the average premium for a 27-year-old making $25,000 a year is 37 percent higher, but the average subsidy rises by 73 percent.

One of Trump's actions was to cut the budget for outreach and education surrounding this open enrollment season by 90 percent.

The Obama Twitter video is part of an effort by nonprofit groups to make up for that loss. A group of former Obama administration officials launched a campaign called Get America Covered to get the word out that open enrollment begins November 1 and runs for six weeks.

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