The Democratic National Committee’s Rules and Bylaws Committee has approved a prospective rule change aimed at preventing non-Democrats from seeking the party’s 2020 presidential nomination. At a DNC meeting in Providence, R.I., the committee embraced a rule change that requires all candidates for the party’s nomination to “run and serve” as Democrats. DNC member Randi Weingarten, president of the American Federation of Teachers, posted a photo of a printout of the rule change in a Twitter message.

The printout reads: “At the time a presidential candidate announces their candidacy publicly, they must publicly affirm that they are a Democrat.” It goes on to say that any candidate pursuing the Democratic Party’s nomination for president must confirm in writing to the Democratic National Committee chairman that they are a member of the Democratic Party, will accept the Democratic nomination and will “run and serve as a member of the Democratic Party.”

This rule seems to be in direct response to Bernie Sanders, the independent Senator from Vermont. Sanders, who describes himself as a democratic socialist, caucuses with the Democrats despite declining to declare a party affiliation. Some Sanders supporters say the move is motivated by “spite” after Sanders fiercely battled Hillary Clinton in a surprisingly close primary race in 2016. He finally endorsed Clinton against Donald Trump about a month after the Democratic National Convention in Philadelphia.

Many Sanders supporters said the DNC tipped the scales against their candidate. Sanders ultimately lost the nomination to Clinton when superdelegates from states he won chose not to vote for him at the party’s 2016 convention. Now, the DNC committee’s action is drawing sharp criticism that the party should be becoming more inclusive, not less.

Mark Longabaugh, a top official in Bernie Sanders’ 2016 campaign, was one of the first from the campaign to comment on the matter. He said, “You know, Bernie Sanders got 13 million votes in 2016. Thousands, if not millions, of those votes were young people and independents he brought into the Democratic Party. And I’m just stunned that the Democratic Party’s rules committee would want to try to make the Democratic Party an exclusive club, for which we want to exclude voters and large segments of the American electorate.”