Trump and Duda had been planning to attend the unveiling of a monument at the Roman Catholic National Shrine of Our Lady of Czestochowa in Doylestown, Pennsylvania on Sunday. The shrine, named after the southern Polish city that is home to a famous Black Madonna painting, is an important site for Polish-Americans.

If the event had gone forward, it would have marked a rare instance of a foreign leader appearing alongside a U.S. presidential candidate on the campaign trail.

It was not immediately clear why the appearance was canceled or if Trump would meet Duda at another time or place.

Voters of Central and Eastern European descent have become highly sought after in the final weeks of the race between Trump and Vice President Kamala Harris, the Democratic nominee.

Pennsylvania, in particular, is home to a sizeable Ukrainian-American and Polish-American population. As the state is among the most competitive in the nation, this demographic could help determine the outcome of the election.

Harris has said that Trump would fail to stand up to aggression by Russian President Vladimir Putin if he wins the November 5 election, a claim the Trump campaign denies.

A Who’s Who of foreign leaders will be arriving in the United States in the coming days for the 79th session of the UN General Assembly.

The Polish embassy in Washington did not immediately respond to a request for comment.