Given how the race for Republican presidential nomination is shaping up in the United States, you might think it's a party where the loudest mouth has a strong chance of winning.

After all, real estate mogul Donald Trump is the front-runner for the nomination, recently securing the support of New Jersey Gov. Chris Christie, himself no slouch in the volume department.

But in fact, argues investigative journalist Jane Mayer, the Republican Party is essentially controlled by the most silent of political players: the Koch brothers.

Charles Koch, left, and David Koch are billionaires who have used their vast wealth to wage a covert campaign to shift the Republican Party towards their far-right libertarian views, says Jane Mayer. (Canadian Press)

"It's really quite something to see because they're just private citizens really," said Mayer, who has written extensively about the brothers in her new book Dark Money: The Hidden History of the Billionaires Behind the Rise of the Radical Right. "They're not people who have run for office."

In an interview with CBC's The National, Mayer says that behind the "spectacle" of the leadership race, the Koch brothers are shaping the party from the inside out.

They get to dominate the conversation and change policies in America by funding a kind of chorus of support for what they want. - Jane Mayer

The brothers — David and Charles — run the second-largest private corporation in the United States. Each is worth an estimated $45 billion. They have used wealth and power to wage a covert campaign to shift American thinking and policies towards their own far-right libertarian views, Mayer says.

"They get to dominate the conversation and change policies in America by funding a kind of chorus of support for what they want."

Mayer argues that the influence of Koch brothers even stretches into Canada, fuelled by their extensive holdings in Alberta's oil sands. Mayer says the Koch brothers have also given hundreds of thousands of dollars to the Fraser Institute, a Canadian think-tank.

The Koch brothers "have very important economic interests in Canada." Mayer said.

You can see the full interview by clicking on the link above or by watching Friday night's National.