Eric Clapton and Robert F. Kennedy Jr. consider themselves rebels in their respective fields -- but those worlds collided in a big way this week in an effort to raise cash ... and a lot of it. The rocker performed at a private show Monday night in L.A. -- where EC plugged in his amp and started strumming at a ritzy estate in Brentwood. No word on who exactly hosted -- but it sounds like the place was packed ... 'cause the show raised $2.2 million. Mind you, this was definitely a political thing ... namely, raising money for RFK Jr.'s presidential campaign -- and Eric was the leading talent and, apparently, the big draw. Tickets for this event were damn pricey -- starting on the low end at $3,300 just for entry and going as high as $6,600 for a little facetime with the candidate himself. Unclear how exactly the ticket sale tiers played out ... but clearly, enough people showed up to 7 figures. We're told that of the money raised -- $1 million went straight to Kennedy's campaign, and the other $1.2 million went toward a PAC that's supporting him. RFK Jr. was grateful for Eric's help in all this, saying ... "I am deeply grateful to Eric Clapton for bringing his musical artistry and rebellious spirit to my gathering." He added ... "I sometimes think that in our divided society, it is music rather than any kind of intellectual agreement that has the most potential to bring us together again. Eric sings from the depths of the human condition. If he sees in me the possibility of bringing unity to our country, it is only possible because artists like him invoke a buried faith in the limitless power of human beings to overcome any obstacle." Eric -- who, like RFK Jr., has openly questioned COVID vaccines, among other things -- was on board from the get-go ... teasing this performance in a YouTube video with some verve.