WASHINGTON - Though he's raking in the cash so far, Barack Obama's decision to forgo public funds for the fall campaign means he must keep up his torrid pace -- a tall order that will tax his time, test his Internet support and require the help of Democratic donors who once wished for his defeat.

The Democratic nominee-in-waiting had his second-best fundraising month in June, a $52 million haul that swamped presidential rival John McCain by more than 2-1. He also got a big boost from his party, which raised nearly five times as much as it had in May.

The new figures underscore the Illinois senator's status as a fundraising star. He has raised $340 million during his presidential run to McCain's $132 million.

Obama's June total also reversed a three-month decline and helped close a cash-on-hand gap between the Democratic and Republican presidential operations. Together, Obama and the Democratic National Committee had $92 million in the bank at the end of June, compared with $96 million for McCain and the Republican National Committee.

But the totals also set a tough new standard for Obama's presidential campaign: The $52 million he raised in June is now a baseline, not a high water mark.

"For him to maintain the pace that it looks as if he will need, he will have to match his best-ever month every month," said Michael Malbin, executive director of the Campaign Finance Institute, a nonpartisan group that tracks trends in political money. "That's what he's done this time, pretty close to his best-ever month, and he'll need to sustain this pace or increase it."

Obama's best fundraising month was February, when he took in $55 million.

Overall, the McCain campaign has estimated that it and Republican Party committees will have $400 million to spend on the presidential election in the months before the November election.

To surpass that level of spending, Obama and the Democratic Party will have to raise about $100 million a month. That task is making some Democrats anxious.

"You don't want to be in a situation at the critical time in September and October when you have not met your budget expectations," said Hassan Nemazee, who was Hillary Rodham Clinton's national financial co-chairman and is now raising money for Obama and the DNC. "You're going to find yourself between a rock and hard place in terms of meeting your numbers."

McCain plans to accept $84 million in public money in the fall -- money he won't have to lift a finger to collect but which will limit his campaign's spending in the fall. The RNC and other party committees will foot the remainder of his campaign bills.