WASHINGTON — President Obama ended March with $104 million in his campaign war chest, 10 times the amount for Republican rival Mitt Romney, underscoring his financial advantage heading into the general election matchup.

Romney, who spent $10.2 million last month beating back a challenge from former Pennsylvania Sen. Rick Santorum, finished the month with $10.1 million on hand, according to campaign finance records filed Friday — a healthy cushion, but small compared with the lead enjoyed by the Democratic incumbent.

The money gap speaks to how the hard-fought GOP primary drained resources from the Romney campaign at a time when Obama has been aggressively gathering money for his reelection bid.

The president’s campaign raised $35 million in March and has pulled in almost $197 million overall, boosted by a network of heavyweight fundraisers who are collecting checks from their friends and colleagues. Eighty-seven new bundlers — including prominent figures such as wellness guru Deepak Chopra, film director Tyler Perry and chef Jean-Georges Vongerichten — joined the finance team last quarter, according to a list released by the campaign.

In all, 532 bundlers are raising money for Obama. Together, they have brought in at least $106 million for his campaign and the Democratic National Committee, including at least $31 million in the last quarter, according to an analysis by the Los Angeles Times Data Desk.

Romney does not release the names of the people bundling money for his campaign.

The former Massachusetts governor has raised a little more than $87 million so far, including $12.7 million in March, and spent the vast majority of his funds battling for the GOP nomination.

Despite March’s dense primary calendar — 22 contests, including 10 on Super Tuesday — the campaign slowed its spending last month, particularly in comparison with Romney’s costly January, when he spent three times as much as he raised. But the pro-Romney “super PAC” Restore Our Future chipped in $11.6 million in ads, phone banking and direct mail on his behalf in March, largely targeting Santorum, his toughest primary opponent.

The former senator suspended his campaign April 10, leaving Romney a clear path to the nomination. Santorum raised $5 million in March, but spent $800,000 more than he raised. He ended the month with $1.8 million cash on hand and nearly $2 million in debt.

The March finance figures underscore Romney’s reliance on Restore Our Future, which is collecting large sums from wealthy individuals and companies. The group raised $8.7 million in March.

The biggest donation came from billionaire energy businessman William Koch, whose brothers finance an array of conservative political causes.

The $1-million contribution was made through Huron Carbon, a subsidiary of Oxbow Carbon, a privately held coal and petroleum supplier that Koch founded and runs. In all, Koch has steered at least $2 million to Restore Our Future: Last year, Oxbow Carbon gave $750,000, while Koch personally gave the group $250,000.

The donations represent stepped-up political activity by the Palm Beach, Fla., resident, whose twin brother, David, and older brother, Charles, are renowned for their support of conservative causes.

“Bill Koch is becoming more engaged because he wants to see entrepreneurship in this country improve,” his spokesman, Brad Goldstein, said Friday. “He has known Mitt Romney for more than three decades and believes Mitt Romney possesses the business skills needed to turn our economy around.”

The Tribune Washington Bureau/Los Angeles Times reported last month that Oxbow Carbon was one of at least five companies with federal contracts that have given money to the pro-Romney super PAC, despite a 36-year-old ban against such companies making federal political expenditures.

Huron Carbon does not have any government contracts, Goldstein said. The company runs facilities that produce calcined coke, an ingredient in the production of high-impact plastics and aluminum.

Other major donors to Restore Our Future in March included San Francisco-based finance figure Charles R. Schwab, the chairman of the brokerage firm Charles Schwab Corp. He and his wife, Helen O. Schwab, each gave $125,000 to the super PAC.

In addition, the Schwabs gave another quarter of a million dollars through Fair Oaks Finance LLC, a private company that gave as its address a location in Hamilton, Mont., that Schwab listed on a license application with the Federal Communications Commission. A call to a spokesman for the Charles Schwab Corp. was not immediately returned Friday.

Priorities USA Action, the super PAC supporting Obama’s reelection, raised $2.5 million in March, ending the month with just over $5 million in the bank. Its biggest benefactor was Amy P. Goldman, a New York-based writer and philanthropist who gave the group $1 million. Comedian Chelsea Handler gave $100,000.

The pro-Obama group, which has struggled to raise money, nevertheless got more in March than American Crossroads, the heavyweight GOP super PAC that was founded in part by Karl Rove. Crossroads pulled in just $1.2 million in March, with the bulk coming from Chicago hedge fund manager Kenneth Griffin, who donated $700,000. (Griffin also gave $850,000 to Restore Our Future.)

But American Crossroads announced Friday that, along with its nonprofit arm Crossroads GPS, it had raised a combined $99.8 million for the election cycle. Of that total, about $71 million was raised by Crossroads GPS, which does not have to disclose its donors.

matea.gold@latimes.com

melanie.mason@latimes.com

Times staff writer Doug Smith in Los Angeles contributed to this report.