The Democratic National Committee warned staff in a Monday memo that they are required to be "impartial and evenhanded" during the party's upcoming presidential primary fight.

There is no clear frontrunner for the nomination like there was in 2016, when the DNC found itself in hot water over its preferential treatment of Hillary Clinton. Former DNC chair Debbie Wasserman Schultz was forced to resign from her post just before the party's 2016 convention after hacked emails revealed favoritism for Clinton among staff.

Current chair Tom Perez is hoping to avoid the same pitfall ahead of the party's next primary, which is expected to be a hotly contested fight between a big field of candidates ranging from sitting members of Congress to wealthy donors.

"As we embark upon the Presidential cycle, the Democratic National Committee (DNC) is committed to conducting itself in a manner that is fair and equitable, so that all bona fide Democratic Presidential candidates have an opportunity to make their case to the American people," read the memo from Seema Nanda, the committee's CEO.

"Pursuant to our Rules, as employees of the DNC, you must remain impartial and evenhanded in the presidential nominating process until a nominee or a presumptive nominee is determined," the memo says. "All staff are expected to avoid impropriety or any appearance of impropriety."

Staff is prohibited, according to the memo, not only from "endorsing or showing a preference towards a candidate," but also from criticizing any of the campaigns.

They are told to avoid "displaying stickers or buttons for primary candidates," "posting on social media endorsing, praising, or criticizing a campaign," and "attending campaign events where your attendance would create the appearance of endorsing a particular candidate."

Staffers were also instructed not to criticize or advocate for any candidate in private correspondence, telling them not to use "DNC communications channels (slack, email, etc.) to support or advocate for the election or the defeat of any particular candidate."

Perez told Huffington Post he hopes releasing the memo "will help ensure that there is no perception of partiality by the DNC."

"The new DNC is committed to making sure that our 2020 nominating process is fair and transparent," said Perez. "These new policies will help ensure that there is no perception of partiality by the DNC during the campaign for the Democratic nomination."

The DNC has spent much of the past two years attempting to repair damage to its relationship with Sen. Bernie Sanders (I., Vt.) and his supporters, who claim they were treated unfairly by the party in 2016. Perez chose a Sanders ally, Rep. Keith Ellison (D., Minn.), to be his deputy early last year, and one of his first actions was to go on a "unity tour" with Sanders.

Sanders, however, has remained a critic. Earlier this year, he took aim at Perez for coming out in favor of New York governor Andrew Cuomo in his primary battle against Cynthia Nixon.

"To endorse one candidate over another is not what the chair of the DNC should be doing," Sanders said, calling the DNC's support for Cuomo a mistake.

The Sanders campaign did not respond to a request for comment on the new DNC memo.