The hecklers showed up eager to make a scene. Dressed in black and pumping their fists in the air, they handed out stickers reading: “Stop L.G.B.T. Aggression.”

But their target, the liberal mayor of Warsaw and a front-runner in a pivotal presidential vote this month in Poland, disappointed them.

The mayor, Rafal Trzaskowski, at a campaign rally in Poland’s conservative rural borderlands near Ukraine, made no mention of gay or abortion rights, or any of the other issues dear to progressives in the Polish capital — and that serve as a red rag to many right-wing residents of the countryside.

Instead, he spoke about the war in Ukraine, the need for a “strong and powerful Poland” and plans to upgrade the military. The crowd waved red and white Polish flags that Mr. Trzaskowski’s team had handed out to ensure that TV cameras framed their candidate in a patriotic tableau.

Much is riding on Poland’s presidential election, the first round of which will be held on May 18, the same day voters in Romania are expected to hand victory in a runoff for the presidency there to a hard-right nationalist and admirer of President Trump.

In Poland, Mr. Trzaskowski hopes to slow Europe’s Trump-empowered tide of right-wing populism — by wrapping himself in the Polish flag, at least at campaign stops outside Warsaw and liberal cities in the west.