WASHINGTON -- Republican Sen. John McCain started June with more than $31 million in the bank after having the best fundraising month of his presidential campaign, according to documents filed Thursday with the Federal Election Commission.

McCain reported raising $21 million in May, his biggest monthly haul yet. He spent $11.6 million during a month and reported nearly $1.3 million in debt. McCain clinched the GOP nomination in early March and has been focused on raising money for the general election.

His campaign had announced the May figure earlier this month. But the paperwork offers more detail on his contributions and spending.

McCain filed the report the same day his Democratic rival, Sen. Barack Obama, announced he would sidestep public financing for the general election and instead rely on his vast network of donors to pump millions more into his presidential bid.

McCain replied by assuring that he would accept the public funds -- some $85 million that will become available after he accepts the Republican presidential nomination at the party's national convention in early September. By accepting the money, McCain cannot raise private funds after the convention.

Despite raising a personal record in May with $21 million, McCain still lags behind Obama. The Democrat had raised more than $265 million as of the end of April. He was expected to reveal his May fundraising on Friday, the deadline for reporting to the FEC.

Counting his May money, McCain has raised about $115 million since he began running last year.

Donors gave $16.6 million directly to McCain's campaign. McCain also raised $4.2 million through a joint victory fund set up with he Republican National Committee.

His major expenses in May were $3.4 million for advertising, more than $1 million for air charter and travel, and $560,000 for payroll costs.