(CNN) - Vice President Joe Biden teams up Wednesday afternoon with one of the biggest Democratic donors: billionaire Tom Steyer.
The vice president will headline a Democratic National Committee fundraiser at Steyer's home in San Francisco.
The former hedge fund manager has become a leading underwriter of Democratic causes and candidates. Steyer, who was a major supporter of President Barack Obama's 2012 re-election campaign, last year spent millions backing the Democratic candidates in the Virginia gubernatorial election and the special Senate contest in Massachusetts.
So far this cycle he's contributed big bucks to Senate Majority PAC, the leading outside group backing Democratic Senate candidates. And Steyer, who's a major opponent of the Keystone XL pipeline and who hopes to make climate change a major issue in the midterm elections, could spend more than $100 million this year on NextGen Climate Action, his super PAC. The group plans to target Republicans in seven Senate and gubernatorial races this year.
As Steyer's involvement in politics grows, so does criticism of him by Republicans. It mirrors the increasing attacks by Democrats on the freewheeling spending of the billionaire industrialist Koch Brothers, who have dished out nearly $40 million so far this cycle backing GOP candidates and causes and attacking Democrats.
Steyer's San Francisco home has been the site of previous fundraisers headlined by the President, Senate Majority Leader Harry Reid and House Democratic Leader Nancy Pelosi.
After the event at Steyer's private residence, the vice president heads to the posh Fairmont Hotel, high atop San Francisco's Nob Hill, to headline a fundraiser for the Democratic Senatorial Campaign Committee.
Biden is scheduled to team up with Sen. Dianne Feinstein of California for the event, which will benefit the DSCC as well as the campaigns of Sen. Kay Hagan of North Carolina and Sen. John Walsh of Montana.
The first-term Hagan faces a challenging re-election this year against North Carolina House Speaker Thom Tillis. Walsh, Montana's former lieutenant governor, was named to the Senate earlier this year to succeed fellow Democrat Max Baucus, who stepped down to become U.S. ambassador to China. Walsh, who is expected to survive a primary challenge next week, should face a very tough election in November against Republican Rep. Steve Daines.
A Democratic official tells CNN that tickets for the event range from $10,00 to $30,000.
The fundraisers in San Francisco come one day after Biden was in Denver to help raise money for another Democrat who faces a challenging re-election, Sen. Mark Udall. In between, the vice president gave the commencement address at the U.S. Air Force Academy.
CNN Political Editor Paul Steinhauser contributed to this report