GDANSK, Poland — As a MAGA-endorsed candidate, he was awarded a visit to the Oval Office, where he posed, thumbs up, with President Donald Trump. Running for president under the banner “Poland first, Poles first,” Karol Nawrocki, a 42-year-old conservative Catholic, narrowly won a June runoff, giving the populist Law and Justice party the ability to paralyze the agenda of pro-E.U. Prime Minister Donald Tusk. As Nawrocki takes office on Wednesday, he joins other Trump-aligned leaders in an increasingly polarized Europe, including Hungarian Prime Minister Viktor Orban and Slovakian Prime Minister Robert Fico. Describing himself as a nationalist and patriot,...