The service is known for its simplicity: It is free at the point of use to anyone who needs it. Paperwork is minimal, and most patients never see a bill. Almost no one in Britain is bankrupted by medical expenses, no one needs to delay medical treatment until he or she can afford it, and virtually everyone is covered.

To take one measure, the United States spends more than all other rich nations on health care, but with decidedly mixed outcomes. According to data from the Organization for Economic Cooperation and Development, the United States spent 17.2 percent of its economic output on health care in 2016, compared with 9.7 percent in Britain. Yet Britain has a higher life expectancy at birth and lower infant mortality.

Why Did Trump Mention It?

Many observers noted that Mr. Trump wrote his Twitter post less than an hour after Nigel Farage — the former leader of the populist U.K. Independence Party and an ally of Mr. Trump — appeared on a Fox News program and asserted that the pressures faced by the N.H.S. were caused by immigration.

“The big problem we’ve got is a population crisis caused by government policy on immigration,” Mr. Farage said. “We have a population of 65 million, but it’s increasing by half a million people a year. We just haven’t got enough hospitals, we haven’t got enough doctors, we haven’t got enough facilities.”

Mr. Farage also warned that if the United States introduced a universal health care system, it would become “politically impossible” to return to a private system or reduce the benefits.

“Let’s be in no doubt we’ve got a big problem, a really big problem with the N.H.S.,” he said.

Britain’s political leaders were quick to answer Mr. Trump. Prime Minister Theresa May said she was “proud” of the service, and Health Secretary Jeremy Hunt, also a member of her governing Conservative Party, responded on Twitter by criticizing America’s health care system for not covering all of its citizens.

Who Favors Such a System?

It depends. Many Americans — Democrats and Republicans alike — talk about support for “universal health care,” but that term usually refers to universal health insurance coverage, and there are competing visions of how to achieve that goal.