WASHINGTON (Reuters) - Donald Trump's joint appearance with Polish President Andrzej Duda in the battleground state of Pennsylvania has been canceled, according to a source familiar with the Republican presidential candidate's plans.

Trump and Duda had been planning to attend the unveiling of a monument at a Polish-American Catholic shrine north of Philadelphia on Sunday. 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 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 Nov. 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.

Trump told reporters on Wednesday that he will "probably" meet next week with Ukrainian leader Volodymyr Zelenskiy, and he has also said he will meet with Indian Prime Minister Narendra Modi.

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

(Reporting by Gram Slattery; Editing by Ross Colvin and Deepa Babington)

By Gram Slattery