A top adviser to Hillary Clinton, Neera Tanden, speculated in a newly leaked email that David Brock, the founder of Media Matters for America, might actually be a "Republican plant" because his efforts to help Clinton were so "counterproductive."

Tanden, president of the Center for American Progress, made the accusation against Brock in an email from January to John Podesta, Clinton’s campaign chairman, the Daily Caller News Foundation reported.

The leaked email was part of a trove of 2000 documents from Podesta’s email account that WikiLeaks published Monday. The document dump came three days after the U.S. formally accused Russia of hacking into political systems to influence the 2016 election.

Tanden sent her email to Podesta after Brock called for Sen. Bernie Sanders (I., Vt.) to release his medical records during the Democratic primary.

"Maybe he actually is a republican plant. Hard to think of anything more counter productive than demanding Bernie’s medical records," Tanden wrote in the email.

Podesta was not happy with Brock’s call for Sanders’ medical records and subsequently tweeted, "@DavidBrockDC chill out."

The exchange shows the level of frustration and infighting within the highest ranks of the Clinton campaign as they tried to figure out how to handle the stronger than expected Sanders during the primary.

Brock used to be a Republican and attacked the Clintons until he switched political allegiances and founded the liberal group Media Matters.