The head of the liberal Center for American Progress privately warned Democratic presidential nominee Hillary Clinton's campaign staff against endorsing a $15-an-hour minimum wage, saying that it would be bad for the economy.

Despite that, both the Clinton campaign and the center have since promoted state and local activists' efforts to push a $15 rate, and Clinton has even said she would sign $15 federal legislation.

Neera Tanden made the warning in an April 2015 email to top Clinton campaign staffers John Podesta, Robby Mook, Jake Sullivan and Jennifer Palmieri. The five were discussing a recent email from New York Mayor Bill de Blasio that gave them a heads up on a rally he was planning that would promote, among other issues, a $15 federal minimum wage. That would more than double the current federal rate of $7.25.

"Substantively, we have not supported $15 — you will get a fair number of liberal economists who will say it will lose jobs. Most of rest seems fine (obviously trade sticks out)," Tanden said in response to de Blasio's email.

WikiLeaks has released a trove of illegally obtained emails from the inbox of Podesta, formerly the head of the Center for American Progress and now Clinton's campaign chairman.

Conservative economists have warned that increasing the rate to $15 would force businesses to cut back on hiring and worker hours to adjust to the higher labor costs. Liberal activists have dismissed the argument, but the email showed that Tanden agreed that it was a bad idea.

Tanden was one of five people appointed by Clinton to the Democratic Party's platform drafting committee in June who voted against the party adopting a $15 minimum wage amendment.

Early data suggests conservatives are right. A study earlier this year by University of Washington economists regarding Seattle increasing its rate to $11 an hour, part of a planned phase-in to $15, found that the increase had not benefited the city's low-wage workers. "Although the minimum wage clearly increased wages for this group, offsetting effects on low-wage worker hours and employment muted the impact on labor earnings," the study found.

The issue nevertheless is popular with liberal activists. Skeptical liberal economists have largely muted their criticism.

The exchange sheds light on Clinton's refusal to formally endorse a $15 federal rate, despite the Democratic Party adopting it as part of their platform. She has advocated an increase to $12 an hour, but has said that if elected she would sign legislation raising the rate to $15 if it landed on her desk. She also has backed state and local activists on the issue.

"I support the local efforts that are going on that are making it possible for people working in certain localities to actually earn $15," Clinton said during a New Hampshire campaign stop in July.

Tanden has tried to get with the movement on the issue. She applauded California Gov. Jerry Brown for signing legislation in March that will make $15 the Golden State rate.

"California's economy is the seventh-largest in the world, and an increased minimum wage will go a long way toward ensuring that all Californians can share in the state's economic growth and prosperity," Tanden said.

A representative for the Center for American Progress could not be reached for comment.

Tanden is president of the center and its sister organization, the Center for American Progress Action Fund, both of which are aggressive, well-funded advocates of liberal issues. It was founded by Podesta, who took a leave of absence from the organization to join the Clinton campaign.