Sen. Sherrod Brown (D-Ohio) was hanging out in a United Airlines airport lounge about a year ago when he noticed something amiss with the bar. The tip jar on the counter, where he would typically drop a few bucks, was gone.

Brown, who likes to stop in these lounges during his weekly travels between Washington and his home in Cleveland, asked the bartenders what happened to the jar. Their answer put Brown in a sour mood before his flight.

“They explained it was corporate policy…. They said, ‘Management told us that it compromises the experience of the traveler,’ whatever that’s supposed to mean,” recounted Brown, who couldn’t remember at which airport he first noticed the change. “Their tips dramatically dropped.... They told me this will cost them in an evening of work $50 or $100.”

The bartenders couldn’t have known how much trouble they just started.

The senior senator from Ohio has his eye on some of the most pressing economic issues of the day ― the rollback of banking regulations, the reshaping of the U.S.-Canada-Mexico trade deal, the possible idling of five General Motors plants, including the one in Lordstown, Ohio. Yet, in spite of his packed Senate schedule, restoring tip jars to United lounges has become something of a personal crusade.

Either Brown or his aides have broached the tip jar policy with United or its lobbyists at least five times since he found out about it. Brown said that includes at least two direct talks between him and United’s chief executive, Oscar Munoz. In some cases, the discussions were primarily about other issues ― for instance, the unionization of United’s catering workers, which United opposed ― but Brown or his staffers have always found a way to bring up the tip jars.

“He had his reasons that didn’t entirely make sense to me, but he was dug in,” Brown said of Munoz. But Brown explained why he wouldn’t let it go. “Whether you’re working in the United club, working at the gate or flying the plane,” he said, “to me, their work should be valued and respected.”

Brown’s persistence hasn’t forced a change in policy yet at United. The airline seemed less than eager to take questions on the matter from a reporter.

“Thanks for your inquiry,” a spokesman responded by email when asked about the policy. “We do not have any policies against accepting tips for our lounge service provider’s bartenders. We just do not allow tip jars to be visible on the counter or back bar. Have a good weekend.”

A follow-up question about the rationale behind the policy went unanswered.

Brown thinks the tip jar spat says something about the balance of power between workers and corporations these days, particularly in the service sector. But it also says plenty about Brown, who is openly mulling a run for president in 2020 but insists he hasn’t made up his mind yet.

In keeping with his Midwestern-labor-Democrat image, the gravel-voiced Brown likes to talk to people about the work they do and how their employers treat them. (In a meeting with 20 reporters last week to talk about a pension crisis for truckers and coal miners, Brown began by asking each reporter their name and where they first worked in journalism.)

The tip jar issue first came to public light last week on Twitter, in a jokey thread between some reporters and Brown’s wife, journalist Connie Schultz. At first, the conversation focused on Brown’s cheap haircuts, which come to $12 and are done by a man named Carlo. That quickly turned to Brown’s tipping habits ― always at least 20 percent, and much more on Carlo’s bargain haircut, according to Schultz.

“Ask him about his ongoing fight to restore bartenders’ tip jars at an airline’s lounges across the U.S.,” Schultz added mischievously.

Sherrod regularly tips *at least* 20% & never leaves a hotel room w/o a $20 tip on the pillow for the housekeeper. He tips those who seldom get tips, eg parking lot attendants. Ask him about his ongoing fight to restore bartenders' tip jars at an airline's lounges across the U.S. — Connie Schultz (@ConnieSchultz) December 6, 2018

Brown jumped into the thread and told of how he’d come to learn of the vanishing tip jars. He unloaded on United, tagging the airline in the thread, and appended his tweet with “SB,” so followers would know he'd personally penned the critique, as opposed to a staffer.

That’s ludicrous. I’ve spoken to @united’s CEO about this multiple times & it’s shameful United hasn’t acted – these workers, and so many other workers around the country, depend on tips. Why shouldn’t customers have every opportunity to fully compensate them for their work? -SB — Sherrod Brown (@SenSherrodBrown) December 6, 2018

United wouldn’t be the first company to prohibit even a subtle encouragement to tip its workforce.

The ride-hailing giant Uber resisted an in-app tipping option for years, to the unending annoyance of its independent-contractor drivers. The company probably knew that a tipping prompt would make customers feel more compelled to tip, which would raise the perceived cost of an Uber ride, something that could diminish Uber’s advantage over taxicabs. The company finally caved in 2017, acknowledging that an in-app tipping option was “long overdue.”

HuffPost previously reported on a unique tipping predicament faced by other airport workers. Many wheelchair assistants were being paid by their employer on what’s known as a tip credit ― meaning they earned below the normal minimum wage, with tips supposedly making up the difference ― but many of the travelers they escorted didn’t know they relied on gratuities to make a living. The workers were forbidden from telling them so. “If we do that, they can fire us,” one worker at Chicago O’Hare International Airport explained.

It stands to reason that workers in United’s lounges would be earning less after their tip jars disappeared. Fewer customers would bother to tip a Starbucks barista if the little tip box weren’t sitting next to the register.

Brown acknowledged that not many workers are affected by the United policy, especially when set against the thousands of buyouts and layoffs at a company like GM. Still, when he thinks about the missing tip jars, he thinks about bigger work issues, like stagnant real wages and the shrinking imprint of unions.

“It may be only 10 people in my state” who are affected by the tip jars, Brown said. “But it says something larger about valuing work and about the dignity of work.”