WASHINGTON (AP) — Deep-pocketed libertarians are giving big to help Rand Paul win the Republican presidential primary.

Three super PACS supporting the libertarian-leaning Kentucky senator said they raised a combined $6 million through June 30. That's on top of the nearly $7 million that Paul's campaign reported pulling in between his April announcement and the end of last month.

"There are some very wealthy libertarians out there, and they're all going to be hearing from me," said Ed Crane, president of one of the pro-Paul super PACs, called Purple PAC. "It's a strong potential base for Rand."

Paul's top backer so far is Jeff Yass, managing director of high-frequency trading firm Susquehanna International Group, who split a $2 million contribution last month between Purple PAC and another pro-Paul super PAC, America's Liberty.

Yass and Crane know each other through the Cato Institute, a free market think tank in Washington that began in the 1970s with the backing of billionaire energy executives Charles and David Koch. Crane is a co-founder, and Yass is a board member.

Joining Yass in the $1 million donor club is George Macricostas, head of a data center company called RagingWire. Those two accounted for most of America's Liberty's $3.1 million in fundraising so far this year.

A third super PAC, Concerned American Voters, has not yet filed a fundraising report, but its leaders said they've raised $1.9 million. Scott Banister, a tech investor in Silicon Valley, is among the donors.

Matt Kibbe, senior adviser to Concerned American Voters and former leader of the libertarian activist group FreedomWorks, said Paul is testing the idea that libertarian donors will make a major investment in a candidate, not just a cause.

"Libertarians are used to giving to ideas and issues campaigns," Kibbe said. "Do they make the leap and give to a liberty-minded candidate with a pathway to victory? I think the answer is yes."

Paul is one of an expected 17 Republican presidential candidates, and he's in the top half of the class for fundraising.