Coined after—and in contrast to—champagne socialist, the phrase prosecco socialist is used as a self-designation by persons with left-wing political views who think of themselves as being better in touch with reality than champagne socialists are.

(The phrase champagne socialist denotes a person who espouses socialist ideals while enjoying a wealthy lifestyle.—Synonym: chardonnay socialist.)

The name prosecco denotes a sparkling white wine from the Veneto region of north-eastern Italy, and the phrase prosecco socialist refers to prosecco as a cheaper alternative to champagne.

These are, in chronological order, the earliest occurrences of the phrase prosecco socialist that I have found:

1-: From Guys like him give socialism its bad name, a review of Philosopher: A Kind of Life, the autobiography of the Canadian-born British professor of philosophy Ted Honderich (born 1933)—review by Robert Fulford, published in the National Post (Toronto, Ontario, Canada) of Tuesday 10^th April 2001:

He’s a socialist who barely tolerates anyone who isn’t and finds Margaret Thatcher ¹ “unspeakable.” Yet he campaigned (unsuccessfully) to persuade Prime Minister Thatcher to install his friend A. J. Ayer in the House of Lords, which Ayer thought his eminence as a philosopher deserved. Even now Honderich doesn’t see that this exercise, a socialist beseeching his enemy for patronage, was inconsistent to the point of nuttiness.
He’s a self-declared prosecco socialist, as opposed to a champagne socialist—the Italian variety being cheaper. Whatever his bubbly, he’s well fixed. Aside from his handsome salary and excellent book royalties, he receives cheques from his multimillionaire brother Beland and admits that all this clashes with his declared belief in equality. When he falls into a calamitous real-estate deal, money from Toronto keeps him solvent. He’s George Orwell’s ² idea of the socialist who gives socialism its bad name.

¹ Margaret Hilda Thatcher (1925-2013) was a British Conservative stateswoman, Prime Minister from 1979 to 1990.
² George Orwell (Eric Arthur Blair – 1903-1950) was a British novelist and essayist.

2-: From I may be a prosecco socialist, but at least I went out to protest, by Charlotte Church ³, published in The Guardian (London and Manchester, England) of Tuesday 12^th May 2015:

I was out demonstrating with the People’s Assembly in Cardiff as those of us who oppose the Tory government must take our message beyond the leftwing bubble.
[…]
For Andrew RT Davies, the leader of the Welsh Conservatives, to describe my exercising of democratic freedom as “unbecoming” really says more than I ever could. […]
As for him, and others, denigrating me as a “champagne socialist”, I have to say I’m more of a prosecco girl, myself. I was born in a working-class family who have for generations been active in political protest. I was nine years old when I was first taken to a demonstration by my mother, who at the time was working as a housing officer for Cardiff council. That was three years before my career as a singer began. I have earned a lot of money from creating music, but I’ve stayed in Cardiff, where my family are, where the people I grew up with are, where my roots are. I could have sacked them all off and moved to LA. I could have made a lot more money by investing in arms and oil, rather than ethically. I could have voted Tory.

³ Charlotte Church (Charlotte Reed – born 1986) is a Welsh singer-songwriter, actress, television presenter and political activist.

The phrase reoccurs in the following from the London Evening Standard (London, England) of Friday 5^th June 2015:

Quote of the day
“He was gross”
Singer and self-proclaimed prosecco socialist Charlotte Church eloquently recounts a meeting with David Cameron ⁴

⁴ David Cameron (born 1966) is a British Conservative statesman, Prime Minister from 2010 to 2016.