Billionaire ex-hedge fund manager Tom Steyer's climate super PAC brought in a bit more than $500,000 from outside donors in July, underscoring the struggles the San Francisco activist has had attracting contributions.

In all, Steyer's NextGen Climate Action PAC has raised about $18.3 million this year, according to Federal Election Commission filings. But that's far short of the $100 million — half his own cash, the others raised from potential donors — he pledged to spend in 2014.

Most of NextGen Climate's funding has come from Steyer.

He made two July contributions worth a total of $7.5 million, bringing his personal investment in the effort to $11.6 million. The other donations in July were a $500,000 donation from Yvon Chouinard, founder of outdoor retailer Patagonia, and $1,000 from Patrick Little, an Auburn, Calif., lawyer.

NextGen Climate, which had $5.8 million of cash on-hand at the end of July, is currently playing in seven states — Senate races in Iowa, New Hampshire, Colorado and Michigan, along with gubernatorial contests in Pennsylvania, Florida and Maine. All the targets are Republicans.

The group has focused on the Iowa Senate race in recent weeks, where GOP candidate Joni Ernst, who NextGen opposes, is leading Democratic Rep. Bruce Braley by 1 percentage point, according to a recent joint poll by the New York Times, CBS News and YouGov.

NextGen Climate did not immediately return a request for comment.