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Senator Bernie Sanders of Vermont has risen in the polls thanks in part to his denunciations of the forces of concentrated wealth, drawing thousands of liberals to hear his jeremiads against corporate power in early nominating states and liberal hubs such as Madison, Wis.

But Mr. Sanders quietly stepped off the campaign trail this weekend to visit Martha’s Vineyard, a favorite summer destination of the country’s elite, in order to mix with representatives of some of the same interests he inveighs against in his stump speech.

Mr. Sanders attended the annual Democratic Senatorial Campaign Committee fund-raiser on the Massachusetts island, a popular gathering that draws some of the most prominent business lobbyists and fund-raisers in the Democratic Party.

One prominent attendee, a supporter of Hillary Rodham Clinton’s presidential campaign, suggested Mr. Sanders’s appearance suggested he was more pragmatic than his rhetoric would let on.

“Bernie is attracting throngs and has a wave going preaching against the one percent,” said the attendee, requesting anonymity to speak candidly. “So why would he take the weekend to spend in Martha’s Vineyard with wealthy people who are donating at least $37,000 and change to the DSCC?” (That is the minimum contribution to the Senate Democrats campaign arm in order to attend the event).

The criticism illustrates the rising irritation among some establishment-aligned Democrats with Mr. Sanders, an independent who caucuses as a Democrat. Some in the party are personally fond of Mr. Sanders, but believe his challenge of Mrs. Clinton, the overwhelming front-runner, is quixotic and will serve chiefly to push Mrs. Clinton to the left and delight Republicans hoping the former secretary of state has to spend money on a primary threat.

Asked about Mr. Sanders’s appearance at the fund-raiser, his spokesman, Michael Briggs, said he would not recalibrate his populist language in the wealthy enclave.

“The people who financially support the Democratic Party need to hear the message that Bernie is giving all over the country and that is resonating all over the country,” Mr. Briggs said. “It is also important that the Democrats take back the Senate.”

Democrats need to win four seats next year, five if they do not win the presidency, to reclaim control of the Senate.