Don’t get duped by moderate Democrats — their message is a loser.

That was the warning Bill de Blasio made to union workers Saturday in Las Vegas, where he urged his party to go hard to the left if it hopes to defeat Donald Trump.

“Do we need to nominate a moderate to win in 2020? You hear it everywhere,” said hizzoner at a forum hosted by the American Federation of State, County and Municipal Employees.

“No, it is a trap, because when you listen to the moderates — I don’t disrespect them, I want to work with them — but I don’t find them inspiring, don’t find them uplifting. I don’t find them visionary.

“I don’t find them to be the people who are going to move all those Democrats who stayed home or don’t feel like things are going to change.”

Coming off his second debate performance and still struggling in the polls, the New York City mayor was stumping hard to make the case for his candidacy.

Rather hizzoner told unions members that they would be better off casting their lot with 2020 candidates who were unabashed progressives … like himself.