During his 10 years on the Senate, Bernie Sanders has been a regular presence at luxurious Democratic fundraising retreats, according to more than a half-dozen lobbyists, donors and former Democratic Senatorial Campaign Committee staff members with whom he attended the events. Sanders most recently appeared at one last July, shortly after he announced his presidential run.

Sanders’ connection to the DSCC has become a issue in his heated primary contest with front-runner Hillary Clinton, who has struggled to explain her close ties to Wall Street and the large speaking fees she’s been paid by Goldman Sachs and other banks. Sanders has made Clinton’s relationships with the financial industry a key point of contrast on the campaign trail.

RELATED: Bernie Sanders suddenly looks like a winner

Clinton has tried to push back, accusing Sanders of both failing to help fellow Democrats and of failing to meet the high-bar he set for himself on being independent of special interests.

The Sanders campaign is all about smashing the alleged stranglehold corporate power has on politics, and the candidate himself frequently touts that his insurgent run is funded by small donations, not wealthy people or outside groups.

It’s been an effective message, and one pro-Clinton forces have tried to muddy by calling attention to Sanders’ ties to the DSCC, which is financed in large part by industry PACs. The DSCC used its financial and political power to bolster Sanders and urge Democrats not to challenge him in his 2006 Senate bid, and spent close to $200,000 through various means to aid his election.

view photo essay The presidential campaign: Bernie Sanders The self-described democratic socialist is known for pushing change on income inequality, college affordability and criminal justice reform.

Bill Clinton said this week that he “practically fell out of my chair” when he read reports that Sanders had attended the DSCC’s summer donor retreats on Martha’s Vineyard, and Hillary Clinton said Sanders took Wall Street cash “not directly, but through the Democratic Senatorial Campaign Committee.”

The Sanders campaign fired back by calling the charge “disturbing,” “dishonest” and “beyond preposterous,” noting the DSCC also receives small donations and is hardly exclusively funded by Wall Street.

Still, the party committee would wither without its largest donors, including industry lobbyists and wealthy individuals, who are feted at semi-annual DSCC retreats on Martha's Vineyard and in Palm Beach, Florida, which always feature a large group of Democratic senators who are brought to schmooze with donors. Sanders has been one of the more frequent attendees since entering the Senate in 2007, including two last year, once before and once after he declared his run.

The private events attract around 150 to 200 guests, according to a former DSCC staff member, about half of whom are lobbyists and half are large individual donors. To attend, guests had to donate the federal maximum (currently $33,400 a year) to the DSCC, or raise $100,000.

Sanders is an independent who caucuses with Democrats. Like the other Democratic senators, Sanders has participated in breakfast meetings and small breakout sessions where one or two lawmakers would meet with groups of guests to mingle and talk shop.

RELATED: Sanders 'not leaving anything on the table' in New Hampshire

Sanders was relaxed and seemed happy to be at the events, according to the attendees, and mostly made small talk, largely steering clear of policy. None said they heard him chastise Wall Street banks, pharmaceutical companies or petrochemical companies -- his frequent targets on the campaign trail -- to the faces of the lobbyists representing those interests, though it's possible it occurred privately.

Some guests said they were surprised to see the populist crusader at these lavish events and suggested he was probably in it for the free vacation.

Senators are flown on a private plane chartered by the DSCC to the retreats, which are held at five-star resorts like the Ritz-Carlton. Sanders was often spotted at the pool, walking on the beach, and at the buffet line. He went on a boat ride off Martha’s Vineyard organized by the committee.

Sanders was even once spotted chatting sociably for close to an hour with a financial services lobbyist who was in a hot tub while the senator sat nearby.

Sanders campaign manager Jeff Weaver brushed off the candidate’s presence at the retreats, noting that guests did not only include financial services industry lobbyists, but also representatives of organized and other progressive constituency groups.

“These are not like Wall Street only events. They represent a much broader range of Democratic supporters,” Weaver told MSNBC.

A guest list obtained by MSNBC from the 2007 Martha’s Vineyard retreat does show a mix of attendees, though labor unions and constituency groups are a minority. “He certainly never lobbied or asked any financial people for money,” Weaver added.

During his 2006 Senate race, Sanders received at least $7,500 directly from lobbyists with ties to the financial industry, including JPMorgan Chase, Citigroup and Visa, as well some energy companies like the American Petroleum Institute.

Weaver dismissed the $7,500 as nothing compared to the $44.1 million he said Clinton has received from donors tied to the financial industry over her entire career. “The truth of the money of the matter is that $7,500 is about equal to 30 seconds of her speech to eBay,” Weaver said of the reported $315,000 Clinton made on a speech to the online auction giant last year.