WARSAW—Like many of his fellow Polish pro-government lawmakers, Dominik Tarczynski is sending a busload of constituents to Warsaw on Thursday to cheer for President Donald Trump. The buses are being provided by a foundation close to the governing party.

“It’s going to be huge—absolutely huge,” Mr. Tarczynski said of the coming welcome for Mr. Trump. “They just love him, the people in Poland—they just really love him.”

Poland...