June 4 (Bloomberg) -- John McCain, the presumptive Republican nominee, raised more than $20 million last month, a campaign adviser said. That was his biggest monthly haul to date.

McCain has been raising most of the money at events in which donors are also being asked to contribute to the Republican National Committee and state parties, the adviser said, speaking on condition of anonymity because he isn't authorized to speak for the campaign.

The Arizona senator raised $18 million in April. He has now raised more than $110 million for his campaign, less than half of the $256 million taken through April 30 by presumptive Democratic nominee Barack Obama.

Since clinching his party's nomination in March, McCain has been asking donors to write a single check to a joint fundraising committee. Contributors would designate $2,300 for his campaign, $2,300 for general election legal and accounting costs, up to $28,500 for the Republican National Committee and as much as $10,000 each to state parties in the potential battlegrounds of Wisconsin, New Mexico, Colorado and Minnesota.

To contact the reporter on this story: Jonathan D. Salant in Washington at jsalant@bloomberg.net.

Last Updated: June 4, 2008 16:50 EDT