Former Maryland Gov. Martin O’Malley took a swipe at Donald Trump on his own turf. The Democratic presidential hopeful lent his support Wednesday to a group of largely immigrant workers trying to organize a union at a Las Vegas hotel owned by the real estate mogul-turned-Republican presidential front-runner.

“There are a couple of things Donald Trump doesn’t understand,” O’Malley said at a press conference outside the Trump International Hotel Las Vegas, flanked by workers. “No. 1, if we want our economy to grow, we need to treat our workers with dignity and respect. Secondly, in every generation, new American immigrants have made our economy stronger and made our economy better.”

O’Malley called for “making it easier, not harder, for people to join unions and bargain collectively for better wages” and passing comprehensive immigration reform. The demands are clear rebukes to Trump. His hotel in Las Vegas has opposed a union drive from the city’s powerful Culinary Workers Union and his presidential campaign has launched vicious rhetorical attacks against immigrants.

The former governor was in Las Vegas for the Nevada AFL-CIO convention. He trails Hillary Clinton and Bernie Sanders in the polls by a large margin.

Trump, on the other hand, comfortably leads the Republican field. A poll released Wednesday showed him trailing Hillary Clinton in a potential general-election matchup by a meager 6 points. Last month, he trailed her by 16 points.