By Michelle Conlin

NEW YORK (Reuters) - The Koch brothers, the most powerful conservative mega donors in the United States, will not use their $400 million political arsenal to try to block Republican front-runner Donald Trump's path to the presidential nomination, a spokesman told Reuters on Wednesday.

The decision by the billionaire industrialists is another setback to Republican establishment efforts to derail the New York real estate mogul's bid for the White House, and follows speculation the Kochs would soon launch a "Trump Intervention."

"We have no plans to get involved in the primary," said James Davis, spokesman for Freedom Partners, the Koch brothers’ political umbrella group. He would not elaborate on what the brothers' strategy would be for the Nov. 8 election to succeed Democratic President Barack Obama.

Three sources close to the Kochs said the brothers made the decision because they were concerned that spending millions of dollars attacking Trump would be money wasted, since they had not yet seen any attack on Trump stick.

The Koch brothers are also smarting from the millions of dollars they pumped into the failed 2012 Republican presidential bids of Newt Gingrich and Mitt Romney, the sources said.

Donors and media reports have speculated since January, when the Kochs gathered 500 of America’s wealthiest political donors at a California resort, that they would deploy their vast political network to target Trump.

The Kochs oppose his protectionist trade rhetoric and hardline views on immigration - which include building a wall along the U.S. border with Mexico and deporting millions of illegal immigrants.

Many Republican figures and business backers are eager to see Trump, a political outsider who has tapped into rising anti-establishment sentiment, fail in his bid for the nomination. They prefer instead a more traditional candidate like U.S. Senator Marco Rubio of Florida.

But with Trump racking up a series of wins in the early nominating contests against opponents including Rubio and U.S. Senator Ted Cruz of Texas, there is a growing sense of inevitability that he will win the party's mantle.

(Editing by Richard Valdmanis, Chris Reese and Peter Cooney)