A hotel union in New York is readying one of the first outside spending campaigns of the Democratic primary for president, promising to commit hundreds of thousands of dollars in the coming weeks to boost the prospects of Mayor Bill de Blasio.

Mr. de Blasio has polled between zero percent and 1 percent in early surveys of the 2020 race and will cling to the outermost edge of the debate stage next week. But Peter Ward, the president of the 40,000-member New York Hotel and Motel Trades Council, which recently endorsed the mayor, is undeterred.

Mr. Ward said Wednesday that the union planned to amplify Mr. de Blasio’s message of fighting for the working class with ad campaigns in three of the key early states: Iowa, New Hampshire and, in particular, South Carolina. The campaign is likely to include digital and television advertising.

“We’re supporting him because, in short, we want to try to help him get on the national stage, talk about working people’s issues, maybe help frame the debate,” Mr. Ward said in an interview. “And, if he gets a little lucky and catches a little fire, more than that.”