The presidential candidate also discussed Cardi B and the genre of Etsy merchandise devoted to him when visiting ABC's 'Jimmy Kimmel Live' on Thursday.

Presidential hopeful Bernie Sanders stopped by ABC's Jimmy Kimmel Live on Thursday to discuss the Robert Mueller investigation, climate change, A$AP Rocky's plight in Sweden and the upcoming Democratic debates.

Kimmel asked the Vermont senator what he thought of special counsel Robert Mueller's testimony before Congress, and Sanders responded with his major takeaway on the hearing: "The message from Mueller … [is that] Russians intervened in campaign in 2016. Whatever your political view is, you want to make sure that the election in the U.S. is controlled by the people of the U.S.," he said.

As for serious reform on elections, Sanders argued, "We have an incredibly do-nothing Senate" and noted that another pressing issue Congress is not confronting is climate change. "We can’t even get a serious discussion about climate change and it is because these guys are owned by the fossil-fuel industry," he said. When Kimmel asked if believers could outspend the fossil-fuel industry, Sanders signaled his disbelief that anything could work short of the "political revolution" the Democrat frequently promises.

After a commercial break, Kimmel got Sanders talking on some cultural topics, starting off with rapper A$AP Rocky being taken into Swedish custody on charges of assault, a move that Donald Trump opposed on Thursday. When Kimmel asked what Sanders planned to do about the news item, the senator chuckled, "Not much at the moment." Kimmel also asked Sanders about his relationship with supporter Cardi B, who Sanders said he had spoken with on the phone a few times. "She is really smart and she is deeply concerned about what is happening in this country. That's the truth, she comes from a humble background ... she wants to make sure that we can improve life for working people in this country," he said.

Next, Kimmel revealed some of merchandise that Sanders has inspired on Etsy, the e-commerce retailer specializing in handmade and bespoke goods, including a sweater showing multiple versions of Sanders howling at the moon. "You seem to be on an all-Bernie volleyball team playing with the moon," Kimmel joked. Another was a Back to the Future parody shirt starring both Sanders and fellow Democratic candidate Beto O'Rourke, and a third showed Sanders as a bird. Sanders explained this had to do with a viral moment during the 2016 campaign when a bird landed on his podium during a Portland rally.

Kimmel noted that Sanders wasn't making any money off of these Etsy products. "Trump wouldn't let that happen," he joked.

Kimmel closed on a more serious note, asking Sanders if he believed that it was "productive" for Democratic candidates to "go after each other, in the way that Senator Harris went after Joe Biden." (Kimmel was referring to Harris' comments on Biden's record on race issues in the first Democratic election.)

Sanders responded with a veiled criticism of Harris: "A campaign is about differences of ideas ... what is not acceptable are ugly personal attacks and trying to destroy people's character. What I hope the debates are about are discussing the very serious issues ... let's not get into personal attacks," he said.

Next week’s Democratic debates will offer a first-time matchup against Massachusetts Sen. Elizabeth Warren and Sanders, following repeated assertions of their friendship while campaigning.

Sanders will hit the debate stage with Warren on July 30, along with along with Minnesota Sen. Amy Klobuchar; former Texas Rep. Beto O'Rourke; South Bend, Indiana, Mayor Pete Buttigieg; Montana Gov. Steve Bullock; Ohio Rep. Tim Ryan; former Colorado Gov. John Hickenlooper; former Maryland Rep. John Delaney; and author Marianne Williamson.