“These are the things that are truly under attack,” he said one day last week, drawing applause from 50 or so labor organizers gathered for an annual conference in the university town of Orono. “The unraveling of the New Deal is just going to continue unless we stand up to them and stop it.”

For at least the past 20 years, whenever a party has won control of the House, it has done so with some kind of unifying message or pitch. In 1994, Republicans ran and won on their “Contract With America,” a 10-point legislative plan. In 2006, Democrats flipped the House with a legislative platform they called “Six for ’06.”

Newt Gingrich, the former House speaker and architect of Contract With America, said the goal was to get insurance or real estate agents entering politics for the first time to memorize 10 winning issues that they could bring to drive-time radio — and “every single issue, the average driver is going to go, ‘Well, I agree with that.’”

Mr. Gingrich argued that Democrats have “no compelling message,” other than that “they hate Trump.”

In fact, many Democrats — including Mr. Allred, Ms. Torres Small and Mr. Golden, all running in districts currently held by Republicans — rarely mention the president by name.

They do not need to, Mr. Luján said: “He’s going to do all the talking about Donald Trump for us.”

Democrats in the House tasked with cobbling together a unifying agenda after the 2016 elections say they studied Contract With America and similar plans in detail. But in the end, they settled on a narrow “do no harm” strategy rather than a granular point-by-point agenda punctuated with a high-profile unveiling at the Capitol. Midterm elections are almost always referendums on the president, regardless of the opposing party’s message.

The Democratic platform, For the People, outlines a relatively anodyne agenda — lowering health care and prescription drug costs, increasing worker pay, cleaning up corruption — that Democrats say unites all of their candidates. (Their anticorruption theme got a lift last week when Representative Chris Collins, Republican of New York, was indicted on charges of insider trading.)