They once sang his praises, but this week these Democrats will grill their former pal, RFK Jr.

WASHINGTON — Before he kept the company of Donald Trump, populist podcast personalities, and fringe conspiracy theorists, Robert F. Kennedy Jr. was a true Democratic insider — a political player who campaigned and fund-raised for his friends, pushed them on policy, and enjoyed a Rolodex befitting a scion of his party’s most beloved family.

In one of the many ironies of Kennedy’s rise to power in Trump’s orbit, it will be his one-time allies — politicians whom he once helped elect and who elevated his work — who could be his most aggressive interrogators in Senate hearings this week over his nomination to be secretary of health and human services.

Senator Sheldon Whitehouse of Rhode Island, who will question Kennedy on Wednesday as a member of the Senate Finance Committee, was his friend and roommate at the University of Virginia Law School. They hunted and fished together; Kennedy once jokingly blamed Whitehouse for his failure to pass the bar exam the first time because the duo went on a whitewater rafting trip, according to a Providence Journal-Bulletin report.

In 1998, when Whitehouse was an underdog candidate for attorney general of Rhode Island, Kennedy was there to campaign with him. “Sheldon is known all over the country for his commitment” to the environment, said Kennedy at a Whitehouse campaign event in 1998 on Narragansett Bay in Cranston, according to the Journal-Bulletin.

When Whitehouse ran for US Senate in 2006, Kennedy went to work fund-raising, describing him in a letter to donors as “a close personal friend and someone who I believe is going to make a terrific Democratic senator from Rhode Island.”

Senator Peter Welch of Vermont, who also serves on the finance panel, also counted Kennedy as an important early political ally. When Welch ran for US House in 2006, Kennedy came to Burlington to stump for him. “This Vermont election is a critical election so we can start taking back the America we love,” Kennedy said, according to the Burlington Free Press.

Other senators came to know and admire Kennedy through his famed advocacy and legal work for clean water, including Senator Ed Markey, of Malden, who serves on the Senate Health, Education, Labor, and Pensions Committee, which will question Kennedy on Thursday.

As a House member in 2008, Markey called Kennedy as a witness for a hearing he convened in a then-operational special committee on climate change. Introducing Kennedy as “one of our nation’s foremost champions for clean water and clean air,” Markey praised him as “a tireless advocate, a prolific author, and a living environmental legend.”

Much has changed in the years since these senators campaigned with and gushed about Kennedy — particularly within the past year, when he left the Democratic Party and became an independent during his quixotic presidential campaign. He later dropped out of the race entirely and endorsed Trump. But even before that, when his activism became defined by antivaccine stances during the Obama years, Kennedy found himself increasingly isolated in Democratic circles; his full-on embrace of COVID conspiracies, along with a number of other fringe beliefs, ahead of his 2024 presidential bid made him an outright pariah.

The deterioration of Kennedy’s relationships with key Democratic lawmakers, however, is a different lens through which to view the well-known story of his flight from liberal royalty to MAGA superstar.

Welch, in a statement to the Globe, noted he met privately with Kennedy last week to discuss his nomination. “The RFK Jr. of 2006 was the Riverkeeper’s RFK Jr.,” said Welch, invoking the name of Kennedy’s environmental advocacy organization. “And it’s clear after meeting with him last week that much has changed since then.”

In response to an inquiry, a Markey spokesperson referred the Globe to the senator’s comments from December explaining his opposition to Kennedy’s nomination.

Spokespeople for Whitehouse and Kennedy did not respond to requests for comment for this story.

“I don’t think it’s going to be a difficult decision for Democrats who had political relationships, even friendships with him,” said Barbara Perry, a professor at the University of Virginia’s Miller Center who has written several books about the Kennedy family. It may not be complicated for these Democrats, she added, because their relationships with Kennedy are “so broken.”

When they encounter Kennedy under the bright lights of his confirmation hearings, these once friendly Democrats are poised to ask him difficult questions about a range of his controversial beliefs and their implications for his leadership of the nation’s crucial health agencies. The Republicans who once viewed Kennedy as a radical liberal crusader, meanwhile, have been replaced with successors keen to follow Trump’s lead.

Most Democratic senators have either ruled out supporting Kennedy or are at least deeply skeptical of him. Markey, for instance, quickly declared he would oppose the nomination, as did Senator Elizabeth Warren. Whitehouse and Welch have not yet publicly said how they will vote; both have been comparatively more muted in their criticism of Kennedy than many of their colleagues.

In December, Welch told the Globe that he was troubled by Kennedy’s views on vaccines but spoke positively of his longtime criticism of big food and agriculture conglomerates and the need for healthier food.

Meanwhile, Whitehouse’s only public statement regarding Kennedy on the platform X was a sarcastic take on a news report from December that a top Kennedy adviser once petitioned the federal government to revoke approval for the polio vaccine.

“Because more polio is definitely what we need,” Whitehouse deadpanned.

On Friday, Whitehouse declined to say how he would vote on Kennedy’s nomination when asked by a Politico reporter. (Still, Whitehouse said in 2023 on ABC that he and Kennedy were no longer close.)

The Kennedy beloved by many Democrats for years was not just the “torch-bearer” of his father and uncles’ legacy, said Perry, but a dedicated activist in his own right. Famously, Kennedy found his passion in environmental law and advocacy after volunteering for clean water groups to satisfy his probation for a drug possession arrest. At his organization, Riverkeeper, Kennedy won court battles to improve water quality and counter polluters on the Hudson River and Long Island Sound.

Through this work, Kennedy crossed paths with many future Democratic lawmakers — some of whom will vote on his nomination soon. When he was attorney general of Connecticut, in the 1980s, Senator Richard Blumenthal, a Democrat, joined Kennedy’s sister group, the Long Island Soundkeeper, in a lawsuit accusing New York state of polluting the water of the sound it shares with Connecticut. They eventually won the case.

In 2009, Senator Kirsten Gillibrand, a New York Democrat, gave a floor speech on Earth Day in which she praised Kennedy and his “extraordinary leadership” on environmental advocacy.

“Bobby Kennedy recognized early on that state and federal environmental legislation cannot only be positive for air, land, and water, but also good for the economy and job creation,” she said, citing their conversations and visit to a school together to tout environmental programs.

A week after that speech, Kennedy issued a critical endorsement of Gillibrand, who was then contending with several possible Democratic challengers who hoped to unseat her from the office to which she had been appointed. “Kirsten has always been an environmental champion,” Kennedy emailed supporters, according to Politico. “If we’re going to finally get serious about climate change and about rebuilding this country, we need Kirsten to keep her spot on the Senate Environment and Public Works Committee.” (Gillibrand ultimately warded off primary rivals and won election easily in 2010.)

Gillibrand’s office did not respond to a request for comment; the senator herself said in December there are a “great deal of issues” she and Kennedy could find “common ground” on if he were confirmed.

Kennedy would trade on his goodwill until his increased propagation of conspiracies began to alienate him from Democratic power centers. In a 2014 story, The Washington Post detailed Kennedy’s considerable ability to easily secure meetings with lawmakers — and the distant reception he got when it became clear he wanted to raise the alarm over his theory that vaccines caused autism. One of those lawmakers was Vermont independent Senator Bernie Sanders, whom the Post described as “polite but noncommittal” regarding Kennedy’s insistence he had science to back up his case.

Still, as recently as 2018, Kennedy was still professing admiration for his longtime allies. Ahead of the 2020 Democratic presidential primary, Kennedy told Framingham’s MetroWest News that the two politicians that most share his father’s values were Markey and Whitehouse.

“I don’t know if any of those people have national ambitions,” he said. “I think we have to see what emerges.”