The Republican National Committee's data arm last year called it a "historic" occasion when it struck a deal to share voter information with the Koch brothers' rapidly expanding political empire.

It was an uneasy détente between the party committee, which views itself as the rightful standard-bearer for the GOP, and the behemoth funded by Charles and David Koch, which is free of the campaign finance restrictions that bind the RNC and plans to spend almost $900 million in the 2016 election cycle to elect a Republican to the White House.

Party leaders, including the current chief digital officer for the RNC, hailed the deal as an important step forward in the GOP's attempt to modernize itself.

But after the fall midterm elections, the deal was allowed to expire without being renewed. Since then, relations between the two sides have soured, turning into what one Republican operative described as "all-out war." Interviews with more than three dozen people, including top decision-makers in both camps, have revealed that the Kochs' i360 platform for managing voter contacts — which is viewed by many as a superior, easier-to-use interface than what's on offer from the RNC — is becoming increasingly popular among Republican campaigns.

The RNC is now openly arguing, however, that the Kochs' political operation is trying to control the Republican Party's master voter file, and to gain influence over — some even say control of — the GOP.

"I think it's very dangerous and wrong to allow a group of very strong, well-financed individuals who have no accountability to anyone to have control over who gets access to the data when, why and how," said Katie Walsh, the RNC's chief of staff.