Vermont Sen. Bernie Sanders put the Democratic Party establishment on notice in his first major address since the Democratic National Convention last month.

"Our campaign ... took on virtually the entire Democratic Party establishment. And I mean the entire Democratic establishment," Sanders said in a speech Wednesday night from Burlington, Vt., launching the senator's "Our Revolution" group meant to continue his progressive campaign past the Democratic presidential primary.

He added that his campaign made the establishment very unhappy, and that's a "good thing."

Sanders praised the people and outreach of his campaign, which he said is "what grassroots success is about." He said that the most important aspect of his presidential run was that he won overwhelming support from young people, 45 and younger, of all different ethnicities.

Though he didn't get everything he wanted, the democratic socialist said the Democrats' platform this year was the "strongest, most progressive" in the history of the country.

"We are going to bring that platform alive, and make it the blueprint for moving the Democrats forward in Congress and all across the country," Sanders said. "We changed the conversations regarding the possibilities of our country."

The Our Revolution group that Sanders helped to launch in the aftermath of his progressive campaign for the White House aims to "reclaim democracy for the working people of our country by harnessing the transformative energy of the "political revolution,'" according to the group's website. One of its main focuses is to mobilize the nearly 13 million voters who voted for Sanders in the Democratic primary, and get other progressive candidates elected into office.

Sanders has already expressed his support for a slew of progressive candidates, including Tim Canova, who is running against former Democratic National Committee Chairwoman Debbie Wasserman Schultz. The Florida congresswoman left her post at the DNC last month after an email leak showed that she was involved in conversation that sought to undermine Sanders' campaign in favor of his rival Hillary Clinton.

Sanders endorsed Clinton in July, and spoke at the convention in Philadelphia in support of the former secretary of state.

Sanders' speech Wednesday follows a report earlier in the week that the majority of Our Revolution's staff resigned over the weekend. Seven key staffers, including members of the organizing and digitals teams, quit in protest after Jeff Weaver, Sanders' former campaign manager, was tapped to lead the group. According to their letters of resignation, obtained by NBC News, the staffers condemned Weaver's "mismanagement" during Sanders' presidential bid. They were turned off by Weaver's interest in turning to TV advertising (instead of online where many younger voters are consuming their media) and large donors, which they said defied the small-donation group they had originally envisioned.

The group, for which Sanders started fundraising earlier in the month, is also facing legal questions about its tax status. Our Revolution is set up as a 501(c)(4) organization, which can receive unlimited contributions. However, because Sanders is an elected official and a former candidate for president, he may be limited in how much he can actually fundraise for the group. While such a group can have some involvement in political activities, its focus should be directed at social welfare.

The senator's wife, Jane Sanders, resigned her chairmanship of the group's board, due in part to the legal implications of the senator being involved with the group.

The senator has also endured some backlash after he and his wife purchased a $575,000 lakefront home and declined to release his personal financial records, after asking for two extensions, during his campaign for the Democratic nomination. While the campaign maintains that it has done nothing wrong, Sanders did run his campaign as one dedicated to transparency in money in politics.