One point that's often lost in heated debate is that immigration could be vital in helping countries to have enough young workers in the economy to support their aging populations.

The bottom line: The control of borders is a serious political problem, but experts are eyeing legal immigration as one solution to a future demographics challenge. As nations age, many will be short of workers to support social programs relied on by the older population.

Robust immigration has buoyed the populations of the U.S., U.K. and other developed nations, keeping them from shrinking for now. But a number of aging countries don't have enough immigration to replace their population as their fertility rates continue to plummet.

The U.S. is unlikely to see any population decline over at least the next couple of decades because of immigration. Sustaining the population is crucial for maintaining strong economic growth and supporting baby boomers' social and health care needs.

Yes, but: "Immigration is no silver bullet by itself: it can slow an aging population, but it would be impossible to reverse it with current or even slightly higher immigration numbers in most countries," Bloemraad says.

• If the expected number of children per woman of childbearing age drops in the U.S. from 1.76 to 1.5 or 1 — well below the replacement rate of 2.1 — no amount of immigration can compensate, Richard Jackson, president of the Global Aging Institute, tells Axios.

Be smart: Demographers report a link between falling fertility rates and the incidence of populism, which in many countries has coincided with strong anti-migrant resistance.

A closer look by country:

• The U.S.: The proportion of immigrants is the highest since 1970, nearing an 1890 record, according to the Migration Policy Institute. The Trump administration, however, has made it harder for high-skilled workers to get visas and proposed stricter penalties for student visa overstays, in addition to the hardline measures it has taken against undocumented immigrants.
• Japan: Immigrants make up only 1.95% of the population, with fertility rates declining and immigration highly unpopular. Its foreign-born population has grown by 40% just since 2013, according to the Migration Policy Institute. But that's still far from the 10% share of the population needed to halt shrinkage.
• Germany has a lower fertility rate than the U.S. or U.K. The country has seen a surge of Syrian immigrants, but anti-migrant politics have followed, threatening Chancellor Angela Merkel's hold on power.
• In China, population growth and fertility rates have fallen significantly since the 1980s — in large part due to its birth limits, which has left the country with an aging population, a shortage of working-age citizens, and millions more men than women. China is now considering ending its birth limits and seeking to attract some high-talent immigrants, the New York Times reported.

What to watch: Mass immigration to developed nations with aging populations would have global economic, social and political impact. The effects would be felt by countries immigrants are leaving in the form of lost human capital, and by those in which they're arriving, with both economic benefits and possible social discord.