Sen. Bernie Sanders' presidential campaign is once again taking aim at Joe Biden for the former vice president’s numerous top-dollar fundraising events from coast to coast.

In a fundraising email to supporters titled “We risk falling behind,” Sanders campaign manager Faiz Shakir wrote Friday that Biden’s “raising huge sums of money at large fundraising events all across the country. And these are not grassroots fundraising events.”

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He said “these are high-dollar functions hosted and attended by corporate lobbyists, health care executives, a Republican casino-CEO, and a union-busting lawyer among others.”

Faiz stressed the independent senator from Vermont's small-dollar and grassroots approach to fundraising, highlighting that “the truth of the matter is that the American people are pretty sick and tired of the billionaire class of this country buying up our candidates and our elections. We can win elections without begging those people for money.”

Biden has been raising big bucks through small-dollar online donations – his campaign this week touted their online contributions and said those kinds of contributions made up the lion’s share of the whopping $6.3 million it raised in the 24 hours after the former vice president announced his candidacy last month.

But Biden's also been holding some high-profile, high-end fundraisers. On his first night as a White House contender, he raised $700,000 at the Philadelphia home of a Comcast executive. He also hauled in big bucks at a Hollywood finance event earlier this month and at two Florida fundraisers this week.

The former vice president’s expected to hold two major fundraisers in Boston on June 5, and two more in New York City on June 17, sources close to Biden’s inner circle told Fox News.

Fundraising was far from then-Sen. Biden’s wheelhouse in his unsuccessful White House runs in the 1988 and 2008 presidential cycles. But so far, the third time appears to be the charm, as Biden’s raking in big bucks both at traditional fundraisers with deep-pocketed donors -- which he's opened up to media coverage in a move for transparency -- as well as through online contributions.

Biden adviser Brandon English touted in an email earlier this week that the campaign’s “fundraising has been driven by rapid, massive growth over the last month.”

The courting of wealthy donors used to be commonplace, but this time around, the two progressive leaders in the Democratic nomination race -- Sanders and Sen. Elizabeth Warren of Massachusetts -- have criticized Biden and sworn off those types of donations.

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But Sanders reportedly has decided to now hold in-person fundraising events and has hired an official to oversee such finance events.