The protests against President Aleksandar Vucic of Serbia had been growing in intensity and size when an unusual guest showed up in its capital this month to meet with the embattled European leader: Donald Trump Jr., the oldest son of President Trump.

The quick visit by Mr. Trump, which included a meeting with Mr. Vucic to talk about U.S. foreign aid to Serbia, came as the Trump family and Jared Kushner, the American president’s son-in-law, were moving ahead with plans to build a Trump International Hotel in Belgrade, the first such property in Europe.

The hotel is slated to be built atop the site of the former Yugoslavian Ministry of Defense headquarters, which was bombed by NATO 26 years ago on land now owned by the Serbian government. Opposition leaders in Serbia have criticized the agreement and called for it to be terminated, raising the prospect that the deal could be scuttled in a change of power.

Mr. Trump used the visit as an opportunity to express his support for Mr. Vucic — a trip that offered perhaps the most explicit mixing so far in President Trump’s second term of U.S. foreign policy and the Trump family’s financial interests.

On Wednesday, the Serbian prime minister resigned, effectively bringing down the ruling party and forcing Mr. Vucic to form a new government or hold new parliamentary elections later this year, creating more uncertainty there.

A spokesman for Donald Trump Jr. dismissed any suggestion that his visit created a conflict of interest. The spokesman said the trip had been driven by a plan to interview Mr. Vucic for Mr. Trump’s podcast, not to step into foreign relations issues or the real-estate deal.