Not taxing tips is a dumb gimmick. Harris is wrong to follow Trump.

To its credit, what was then President Joe Biden’s reelection campaign, which also needed to carry Nevada, refused to match what it called Mr. Trump’s “wild campaign promise.” It said there were more effective ways to reach the same goal. Lael Brainard, director of the White House National Economic Council, said tipped workers in Nevada would gain an average $6,000 from eliminating the federal tipped minimum wage and raising the general minimum wage.

Much has changed since then. Mr. Biden is gone from the Democratic campaign and so is rationality about this issue. On Saturday, the new nominee, Vice President Kamala Harris, promised — in Las Vegas, naturally ― that she, too, would “eliminate taxes on tips for service and hospitality workers.” On Monday, White House press secretary Karine Jean-Pierre said the president would sign such a law.

Presidential candidates have long campaigned in Iowa’s caucuses by pledging to support wasteful ethanol subsidies. Now, apparently, ending tip taxation is the ethanol of Nevada, a state whose six electoral votes could decide the presidency. About 300,000 people work in casinos, hotels and restaurants there; Mr. Biden won by 33,596 votes in 2020.

The tax code treats gratuities the same as regular wages. In 2018, the most recent year for which data is available, 6 million taxpayers reported roughly $38 billion of tips, according to the IRS. Of course, many workers don’t report cash tips, which makes them de facto tax-free. But expanded use of credit cards and mobile payments for tipping has made it harder to hide tip income from the tax man — and created an issue for Mr. Trump and, now, Ms. Harris, to exploit.

Even so, it’s an overreaction: Of an estimated 4 million tipped workers in the United States in 2023, more than one-third earn so little total income that they don’t owe any federal income tax, according to Yale’s Budget Lab. Many are eligible for the earned-income tax credit, a federal wage supplement. (For technical reasons, they might lose that if their tips are no longer subject to taxes.) As for the impact on federal revenue, that depends on how widely the exemption extends. If applied to income tax and Social Security and Medicare payroll taxes, as a bill by Reps. Matt Gaetz (R-Fla.) and Thomas Massie (R-Ky.) would do, the loss to the Treasury could be between $150 billion to $250 billion over a decade, according to the Committee for a Responsible Federal Budget. The figures would be about half that if tips were exempt from federal income taxes only and a little bit less if this exemption phased out for higher earners, CRFB estimated.

Of course, if workers and businesses try to shift compensation to tips, the revenue loss would be greater. And, because incentives influence behavior, they probably would try: Ready to tip the guy at Jiffy Lube? Your dental hygienist? Ms. Harris’s campaign says her proposal would retain the payroll tax on tips; she’d seek “an income limit and with strict requirements to prevent hedge fund managers and lawyers from structuring their compensation in ways to try to take advantage of the policy.” The Trump campaign has so far not provided specifics on these questions.

No matter the details, the whole idea of tax-free tips rests on an arbitrary distinction among workers. Why should a warehouse stocker, a grocery store checker, a security guard or a sanitation worker need to pay taxes on all their income but not a Black Jack dealer who earns most of her money from “tips”? The Tax Foundation highlighted the unfairness by comparing a hypothetical restaurant server and a cashier who each make $34,000 annually. Under current law, each pays about $2,100 in taxes. But without a tax on tips, the server could get a tax cut of more than $1,600.

In 2025, Congress must decide how and whether to extend the expiring 2017 individual tax cuts. Pandering by the presidential candidates has ensured that the first bipartisan policy idea to affect that debate will be a bad one.