Some members of the American Federation of Teachers are furious over what they say was a premature and undemocratic endorsement of Hillary Clinton, and are now calling for the teachers union to withdraw its support in light of Vermont Sen. Bernie Sanders' growing popularity.

"There was no internal discussion. Zero. Zip," said Steven Conn, president of the Detroit Federation of Teachers, reports the New York Post. "This is wrong and something needs to be done."

The 45-member AFT executive board voted Saturday to endorse Clinton, becoming the first international labor union to back a candidate in the 2016 presidential contest, Politico reported. AFT is one of the 56 unions that makes up the AFL-CIO.

The problem is, labor leaders say there was an agreement that unions would hold off on endorsing candidates until after July 30, when all of the Democratic candidates will have an hourlong interview at AFL-CIO headquarters, giving union officials a chance to explore candidates' positions on controversial issues such as trade.

"A request was made, and there was an expectation that people were going to at least allow the AFL-CIO process to proceed," one labor operative told Politico. "When the AFL-CIO was asking people not to make endorsements, why did they feel the need to do it in such a hurried fashion?"

Other labor leaders said it was "an insult" to endorse Clinton now, particularly because, unlike Sanders, Clinton has refused to take a clear position on trade.

Politico points out that AFT's president, Randi Weingarten, is a longtime friend and donor to Clinton. She has given money to Clinton super PACs, serves on the board of the pro-Clinton Priorities USA Action super PAC, and introduced Clinton when she announced her bid for Senate in New York in 2000.

That being the case, critics say the early endorsement was likely an attempt to hamper a surge by Sanders, Clinton's main rival, who is generally seen as a champion of labor issues and has earned support from labor leaders in states like Iowa, South Carolina and Vermont.

While Clinton still leads the Democratic field, from April to July, support for Clinton among Democrats and Democratic-leaning voters dropped 11 percentage points, while support for Sanders went up by 10 percentage points, according to Monmouth University. That increase in support for Sanders is another reason people are questioning the timing of the endorsement.

During the Democratic presidential primary in 2008, the teachers union waited until October 2007 to endorse Clinton.

Nearly 4,500 teachers have signed a Change.org petition calling for AFT to withdraw its endorsement of Clinton.

The petition cites the following reasons:

"1. Hillary Clinton is on record for supporting the expanded use of charter schools, which directly and negatively impacts public education.

2. Clinton also is in favor of tying teacher pay to student performance on standardized tests. These tests are proven to be biased and an inaccurate measure of student achievement.

3. Other candidates are much more teacher friendly. For example, Bernie Sanders is a product of public education and wants to help teachers teach in a holistic way, not just teach toward a test."