Demonstrator Zev Nicholson, of Boston, left, protests outside a McDonalds, March 18, 2014, in Boston. (Steven Senne/ASSOCAITED PRESS)

Ronald Klain, a Post contributing columnist, served as a senior White House aide to Presidents Barack Obama and Bill Clinton and was a senior adviser to Hillary Clinton’s 2016 campaign.

There’s energy in the Democratic Party around big ideas, such as single-payer health care or a universal basic income. But the most significant thing Democrats could do to help working families right now isn’t designing a grand new program — it’s getting in the trenches to fight for a simple old idea. Specifically, trying to stop the Trump administration from denying millions of workers the overtime pay they have earned.

The idea that employees should receive time-and-a-half for their work beyond 40 hours a week dates to the 1938 Fair Labor Standards Act. But not all workers are entitled to overtime. The law allows employers to “exempt” workers who have salaried jobs performing “executive, administrative, or professional duties.” The Labor Department, which administers the law, has also set a salary ceiling above which workers — even in low-level supervisory roles — are ineligible for overtime pay. Today, that limit is woefully outdated. In 2016, salaried workers making only $23,700 a year — the equivalent of about $11 an hour — could be classified as “executives” or “professionals” and thereby ineligible for overtime.

Who loses overtime pay when the cutoff is so low? Front-counter managers at fast-food outlets; head cashiers at big-box stores; crew chiefs on job sites; night-shift leads at convenience stores. These people toil a million miles from the C-suite, and struggle to get by on as little as $455 a week. But they are labeled “exempt” and can be required to work 60 or 70 hours a week without any compensation for extra hours: They are paid a flat salary and are ineligible for overtime pay. Many are single parents, paying dearly out of pocket for child care when they are forced to work extended shifts.

In his final year in office, President Barack Obama acted to fix this travesty. In May 2016, he raised the floor for overtime pay from $23,700 to $47,500 effective Dec. 1. Of course, that number is still low: someone making just $48,000 would still be ineligible for overtime under the new rule, even though he or she is hardly a quintessential “executive.”

Before the rule took effect, a court froze its implementation, and a self-proclaimed “champion” for working people won the presidency. Observers wondered: Would Donald Trump take up Obama’s fight for these hard-pressed, hard-working people (many of whom voted for Trump) and insist that they be paid overtime?

Nope. Rather than siding with his voters, President Trump caved to major employers. Last month, his Labor Department announced that it was considering “stakeholder” — read: business — concerns that the new rule’s salary level was “too high.” Attention MAGA red-hat wearers: The president believes if you are making $600 a week, you may be making too much to deserve overtime. The Lord of the Apprentice Boardroom, the King of Mar-a-Lago, the Master of Trump Tower thinks you — making $15 an hour — may be paid too much to get overtime. He believes your boss should be able to make you come to work at dawn, stay far into the night, and not pay you one dime more for the extra hours worked.

This is no minor issue. According to the Economic Policy Institute, which has been leading the fight for overtime reform, revoking the Obama rule could affect the paychecks of more than 12 million workers.

Keep in mind that making workers eligible for overtime pay does not require any employer to pay overtime. A company can simply hire more workers, and spare existing employees the burden of working extra hours. Standard Republican objections to pro-worker measures don’t apply to the case against overtime. Enforcing overtime rules doesn’t raise taxes. It doesn’t “destroy jobs” — if anything, it should create more. It doesn’t give undeserving people “handouts.” It is simply fair pay for long hours.

Democrats should relish this fight. On Thursday, a federal district judge in Texas, in a lawsuit brought by business groups, struck down the Obama rule as contrary to a 1961 statute; Democrats should challenge Donald “Friend of the Working Class” Trump to appeal the decision.

Every billionaire member of the Trump economic team who testifies on Capitol Hill should be grilled over whether they agree that $47,000 a year is “too high” to receive overtime — and asked how few hours they worked in the private sector to earn the same amount these workers make in an entire year. Democrats should try to attach amendments to legislation, updating the law the Texas court relied upon, and putting the Obama rule into effect. They should talk about overtime whenever they can, and challenge Republicans on it in every corner carryout, gas station, work site and suburban mega-store.

Overtime may not be a sexy issue; it’s not a “new idea.” But it’s hard to imagine a more worthy battle for a party wrestling with Trump over who really stands for working men and women.