"The decline is partly coming off high levels of the prior year, but also surely from the strong rhetoric coming out of Washington against foreigners," said Lawrence Yun, chief economist for the Realtors. "There has been a large drop-off in foreign students attending U.S. universities already. Chinese [buyers], in particular, purchase homes for their kids while attending college."

While high home prices and inventory shortages are clearly playing some role in the drop. Competition from domestic buyers, whose demand is increasing sharply, may also be a deterrent. And the current political climate in the U.S. also should not be overlooked.

Of the $121 billion in sales to international buyers, those currently living in the U.S purchased $67.9 billion in properties, while nonresident foreigners purchased $53 billion, both marking a drop from the previous year. Foreign buyers accounted for 8 percent of the $1.6 trillion in existing home sales, a drop from 10 percent the previous year.

The dollar volume of U.S. home sales to international buyers between April 2017 and March 2018 dropped 21 percent compared with the year-ago period, according to the National Association of Realtors.

After strong interest for several years, international buyers appear to be souring on the U.S. housing market.

China still leads the pack for international buyers, as it has for six straight years, accounting for 15 percent of international sales. Chinese buyers also purchased the most expensive homes, with a median price of $439,100.

Canada came in second, with a 10 percent share of international sales, but the Canadians' dollar volume dropped by 45 percent compared to the previous year. Not only are Canadians buying fewer U.S. properties, they are buying cheaper U.S. properties. The median price for Canadian buyers was $292,000.

"The market here is softer, and I imagine that's why there are perhaps less Canadian buyers," said Elli Davis, a real estate agent based in Toronto. "That does surprise me, though, as I still know lots of people buying mostly in Florida!"

Buyers from China, Canada, India, Mexico and the United Kingdom accounted for nearly half of the dollar volume of sales to international buyers. Canadian buyers had been the market leaders by far during the U.S. recession. They dropped back significantly as U.S. home prices recovered, Chinese buying increased and U.S. investor purchases climbed.

"Inventory shortages continue to drive up prices and sustained job creation and historically low interest rates mean that foreign buyers are now competing with domestic residents for the same, limited supply of homes," Yun said.

High prices could certainly be a deterrent for buyers in Southern California. Chinese buyers have been very strong in the single-family market there, as they plan for their children to attend area universities. Irvine, especially, saw huge demand from Chinese buyers, particularly in newly built communities, with larger, multigenerational homes that they favor.

For international investors who are looking for condominiums in large cities as an investment, the supply theory doesn't really hold.

"I don't think it's the supply issue because these buyers are buying in the higher end and there is more supply there, particularly in the gateway cities like Miami and New York," said Sam Khater, chief economist at Freddie Mac. "It could be just that their appetite for U.S. real estate is waning."