Ed Markey helped make Elon Musk. Now, Markey is working to slow him down.

Elon Musk (left) and Ed Markey.

Last November, when Senator Ed Markey sent a letter to Elon Musk expressing concern about Musk’s governance of Twitter and the havoc that fake verified accounts were wreaking, he didn’t just e-mail it to the company. Markey tagged Musk on the platform the billionaire owns.

Naturally, that’s also where the only reply came — and where a feud was born. Musk was snarky and dismissive, mocking Markey as sounding “like a parody.”

It was one of the few public interactions the pair has had. But their history goes far beyond trading dunks on Twitter, the platform Markey has adopted as his signature communication method. For while he mocks the senator now, Musk has benefited from Markey’s environmental foresight — policies that Markey got through Congress helped boost the electric vehicle market, build Musk’s wealth, and make him a household name.

And Musk, in turn, has helped Markey realize some of his dreams as an elected official. Musk’s successful management of the electric car startup Tesla helped make the wide availability of emissions-free cars closer to reality. But that doesn’t mean Markey has hesitated to take Musk on, either about his car company or, now, Twitter.

“I praise him for the breakthrough he made in electric vehicle commercialization; I praise him for what he did on Space X,” Markey said in a recent interview, referring to Musk’s rocketry venture. “But when he purchases Twitter ... he’s now dealing with democracy, and that’s much more complicated than anything he’s ever done before in his life. ... He has to know that we’re going to oversee his activities at Twitter to make sure that he brings this civic mindedness to all of his other ventures.”

Tesla was already a growing company when Markey wrote key parts of the Energy Independence and Security Act of 2007, including a provision to raise fuel economy standards for the first time since the 1970s, requiring car companies to manufacture vehicles that averaged 35 miles per gallon by 2020. President Barack Obama built on those standards two years later, passing regulations that would have pushed the average to 54.5 miles per gallon by 2025, though that mandate has since been rolled back.

Tesla produced its first car model, the Roadster, shortly thereafter, in 2008, but was teetering on the edge of bankruptcy.

The new fuel economy standards raised interest in electric vehicles, at the same time that Tesla needed investment. Further, another provision of the law allowing car companies that exceed fuel standards to sell carbon credits to those that do not meet them provided another revenue stream for Tesla. With its fuel-free cars, Tesla had plenty of credits to sell.

Musk also benefited from another part of the law, supported but not written by Markey, that created a loan program to spur automotive innovation, from which Tesla received a loan of almost $500 million in 2010. Tesla paid the money back to the Department of Energy with interest in 2013, but the infusion of cash came at a critical time for the company as it was getting its now sought-after cars to market.

Industry experts note that plenty of Tesla’s success is due to its sophisticated design, smart marketing strategy and innovative engineering. But they say it’s hard to separate the boost that came from the Markey-led environmental policies.

“They absolutely could not have been successful at the critical early stages of their existence without the infusion of money that came from selling credits and the encouragement they got from other California laws and regulations as well,” said Mary Nichols, a former top climate regulator in California who had a front-row seat to Tesla’s rise. “Government was absolutely a part of getting them to come into existence and create a toe hold before they were a success.”

“Tesla would have a tough road on electric vehicles, if Ed Markey hadn’t cleared a path for it,” said Dan Becker, a longtime climate advocate now with the Center for Biological Diversity. “The success of electric vehicles is dependent in large part on Ed Markey and a few of his colleagues who changed the law to require the automakers to change the way we do things”

November’s public spat was not the first time Markey has turned the spotlight on Musk, having spent years questioning the safety of the self-driving technology Tesla developed and querying Tesla’s record. Markey has sent letters to automakers probing autonomous driving, pressed federal regulators to investigate the technology after deadly crashes, and introduced legislation that was included in a major infrastructure bill to study self-driving systems.

But it was his fondness for communicating on Twitter — he was an early adopter of social media platforms and prides himself on knowing how to communicate with younger activists — that put him on a particularly public collision course with Musk.

Markey twice picked a fight with the mogul by letting a Washington Post reporter impersonate him on the social media platform to demonstrate how easily manipulated the site was under Musk’s leadership.

The two sparred in a flurry of messages in November, with Musk retorting, “Perhaps it is because your real account sounds like a parody?” and mocking Markey for wearing a mask. Markey fired right back.

“One of your companies is under an FTC consent decree,” Markey tweeted. “Auto safety watchdog NHTSA is investigating another for killing people. And you’re spending your time picking fights online. Fix your companies. Or Congress will.”

Musk nevertheless shrugged Markey off. Responding to a progressive activist group that reminded him that Markey sits on the Commerce Committee with oversight of most of Musk’s companies, he tweeted, “Are you suggesting the Senator will abuse his political power to attack me?”

In an interview, Markey insisted that he’s all for the Twitter platform, just with appropriate management.

“I love Twitter,” Markey exclaimed. “It’s where so many of my friends and grassroots allies gathered to advocate for the Green New Deal, LGBTQ rights, so many other important issues, but we just have to make sure that it’s a safe town square for everyone to be able to coexist.”

Neither Tesla nor Twitter was available for comment; neither company has an active communications office.

Markey isn’t done with Musk, far from it, and on Friday he sent a letter to Musk about his concern that the platform is no longer accessible for people with disabilities, another longstanding area of work for Markey.

In the letter, shared first with the Globe, Markey wrote that Musk’s firing of Twitter’s Accessibility Team when he took over the company last year eliminated important services for disabled users, including automatic closed captioning on Twitter Spaces and policies that will block third-party apps that added ways to help understand features such as images.

“Twitter’s historical record on disability access was far from perfect, but in recent years, the Accessibility Team markedly improved it,” Markey wrote. “Your decision to eliminate Twitter’s Accessibility Team therefore represents a dramatic and unwelcome shift, one that has already had devastating consequences for disabled Twitter users.”

Markey’s relationship to Musk, at times both supportive and antagonistic, reflects the broader arc of the Massachusetts Democrat’s career. The 76-year-old has long been at the vanguard of the environmental movement and is excited by novel technologies. He still keeps on his office wall the digital pen that President Bill Clinton used to sign his landmark 1996 telecommunications bill.

But Markey has also developed a reputation as a consumer watchdog, on everything from children’s privacy to disability advocacy, a passion, he said, that stems from his childhood in Malden. Growing up in the blue-collar suburb during the emergence of the state’s high-tech industry gave him a deep commitment to ensuring economic advancement doesn’t come at the expense of fundamental values, he said. Those values sharpen Musk’s role as both a hero and villain for Markey.

Musk, meanwhile, remains a formidable adversary. He wields a powerful megaphone, with nearly 130 million followers on Twitter and a staunch constituency on the right as his politics have become increasingly conservative.

Markey’s colleagues and longtime partners say Musk should also know who he’s facing, a power in the Senate with a platform of his own, and a knack for hatching the right catch-phrase or sales pitch at the right moment.

Joan Claybrook, a former head of the National Highway Traffic Safety Administration and longtime president of the consumer advocacy nonprofit Public Citizen, said she perceives some parallels between Musk and Markey, which may be what make then so well-matched.

“Ed is as independent in his field as Musk is in his, and that’s Markey’s complete strength,” Claybrook said. “So he’s a good overseer, because Musk can’t take him down. ... Ed comes across as kind of a simple guy from Malden, Massachusetts, but he’s not.”