The US Republican Party threatened to prevent two US television networks from carrying party primary debates if they don't abandon planned documentaries on Hillary Clinton.
NBC Entertainment is planning a miniseries and CNN is working on a documentary about the Democratic former secretary of state as she mulls a potential run for the White House in 2016.
"As an American company, you have every right to air programming of your choice," Republican National Committee chairman Reince Priebus wrote to CNN president Jeff Zucker and NBC Entertainment chairman Robert Greenblatt.
"But as American citizens, certainly you recognise why many are astounded at your actions, which appear to be a major network's thinly veiled attempt at putting a thumb on the scales of the 2016 presidential election."
Clinton, a former first lady and US senator whose turn as America's top diplomat earned her widespread praise, is seen as the clear Democratic frontrunner for president.
She has made no announcement of her political plans but the RNC letters - more than three years ahead of the 2016 election - show the party's nervousness about a run by Clinton, who narrowly lost the Democratic nomination to Barack Obama in 2008.
"Your credibility as a supposedly unbiased news network will most certainly be jeopardised by the decision to show political favoritism and produce an extended commercial for secretary Clinton's nascent campaign," Priebus told Zucker.
In a separate statement, Priebus blasted the networks, saying "their actions to promote Secretary Clinton are disturbing and disappointing."
Should CNN and NBC not agree to pull the productions before the RNC holds its summer meeting beginning August 14, Priebus told the networks he will seek a binding RNC vote "stating that the committee will neither partner with you in 2016 primary debates nor sanction primary debates which you sponsor."
CNN responded by urging the RNC in a statement to not make "premature decisions about a project that is in the very early stages of development and months from completion."
The broadcaster also said it would be odd if the RNC declines to host Republican debates, saying that limiting such participation would "be the ultimate disservice to voters."
NBC issued a statement simply saying "NBC News is completely independent of NBC Entertainment and has no involvement in this project."
NBC Entertainment announced in July it was producing Hillary, a four-part miniseries starring Diane Lane.
CNN's film division said it has commissioned a feature-length documentary by Academy Award-winning director Charles Ferguson that will run in theaters in 2014 before airing on the cable station.