Tom Steyer, the former hedge fund executive who pledged to raise $100 million this year to elect candidates concerned about climate change,will host a Democratic party fundraiser headlined by Vice President Biden this month at his San Francisco home.

The May 28 event will be the fourth party fundraiser in which the focus of the conversation has been on climate, an adviser said. Democrats are looking to Steyer even as they criticize the Koch brothers for providing large amounts of cash to GOP candidates who have a skeptical view of federal environmental regulation.

Previous fundraising events at Steyer’s home have featured President Obama, Senate Majority Leader Harry Reid (D-Ky.) and House Minority Leader Nancy Pelosi (D-Calif.).

The details of the May 28 event, released Monday, were yet another sign that the hedge fund investor is emerging as a leading underwriter of Democratic candidates. Some party operatives were rattled by suggestions from Steyer last year that he might target candidates who support the Keystone XL pipeline, which would carry oil-rich tar sands from western Canada to refineries along the Gulf Coast. Some Democrats, including Sen. Mary Landrieu of Louisiana, have said they favor the project. Environmentalists dispute the number of jobs that Keystone advocates say will be created by the project.

In addition to the upcoming fundraiser for the Democratic National Committee, Steyer has donated $5 million to the Democrats’ Senate Majority PAC, a contribution first reported last week by the New York Times. On Wednesday, staff working for Steyer and his NextGen Climate Action Super PACS are conducting a private briefing with a select group of Washington reporters to provide more detail about the billionaire investors plans for 2014. Earlier, Steyer said that he would provide half of his projected $100 million budget for this campaign year from his own personal fortune and raise the rest from outside donors.