The Republican Party has threatened to boycott whichever television channel broadcasts a proposed mini-series on the life of possible Democratic candidate Hillary Clinton.

The United States is still three years out from its next presidential election but Republicans fear that the proposed mini-series - yet to have a script - could unfairly influence voters come election time.

A four-part NBC miniseries starring Diane Lane and a feature-length documentary on CNN are set to air in 2014.

Republican National Committee chairman Reince Priebus is threatening to keep presidential debates involving Republican candidates off the channels if they go ahead with what he says amount to political ads for Ms Clinton.

Mr Priebus has written to NBC and CNN executives, objecting to what he called "a thinly veiled attempt" by major networks "at putting a thumb on the scales of the 2016 presidential election".

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"Listen, I'm not going to boycott Diane Lane. It's not her fault that she's decided to take a script," he said.

"The fact is, what channel am I going to tune in to see the documentary and the miniseries that is all about promoting Hillary Clinton?

"And at this point, it sounds like it's going to be CNN and NBC, and if that's the case, they're not going to be involved in our debates. Period."

Two of NBC's own correspondents - White House chief correspondent Chuck Todd and veteran broadcaster Andrea Mitchell - have also spoken out, suggesting broadcasting the miniseries could damage the reputation of NBC News.

Neither CNN or NBC seems inclined to pull the plug.

NBC says the miniseries will air before the heat of the campaign and that the NBC News unit is completely independent of NBC Entertainment and had no involvement in the miniseries.

In a statement, CNN called the Republican actions "premature", saying "should they decide not to participate in debates on CNN, we would find it curious as limiting their debate participation seems to be the ultimate disservice to voters".

Political commentator Ron Bonjean says the Republicans are misguided in targeting the media.

"If I were the Republicans, I would be focusing on building up their own candidate, making sure their own candidate is someone who is going to connect with people, and someone who's going to be on-message, and someone who could take on Hillary Clinton," Mr Bonjean said.

Ms Clinton still has not said she is running, but many political analysts expect she will and her every move - including a recent lunch with Barack Obama - is already making news.