It took more than 90 minutes for the moderators of the Democratic presidential primary debate on Wednesday night to turn to the economy, which polls show is a top issue on voters’ minds and is one of President Trump’s strengths as he seeks re-election.

The candidates wasted no time attacking him on it.

Over two nights of debates in Detroit, candidates assailed Mr. Trump’s record on trade, tax cuts and wage growth, accusing him of perpetuating economic inequality and a “rigged” system that favors the wealthy and powerful. It was a preview of the economic arguments that are likely only to grow as the 2020 race escalates.

“Donald Trump came in making a whole lot of promises to working people that he did not keep,” Senator Kamala Harris of California said. “He said he was going to help farmers. He said he was going to help autoworkers.”

Ms. Harris said that farmers were now staring down bankruptcy, that hundreds of thousands of autoworkers could soon be out of work and that the chair of the Federal Reserve, Jerome H. Powell, had just “admitted” that the central bank cut interest rates by a quarter point on Wednesday because of economic uncertainty created by Mr. Trump’s trade wars.