(CNN) - Vice President Joe Biden called campaign donors to action on Saturday, warning them that Mitt Romney's campaign was out-raising the president's reelection effort.
"Building this campaign today is more important than it was a few days ago," he wrote in an e-mail to supporters. "We can still win even while getting outraised by these guys. But we've got to keep it close."
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Romney and his allies brought in over $100 million in June, sources said on Thursday.
President Barack Obama's reelection campaign has not yet released June numbers - and is not required to do so until later this month. But if the Democrats' haul is less than Romney's, it would be the second straight month he was out-raised by the Republican.
In May, Romney and allies brought in $77 million, with Obama and his allies raising $60 million.
Obama campaign officials predicted at a background briefing with reporters in June that they would be outspent by the Republican effort this year.
Biden's e-mail - bearing the subject line "A big problem right now" - came the day after Obama campaign manager Jim Messina sent a message to supporters titled "Urgent."
"If we don't take this seriously now, we risk finding ourselves at a point where there is too much ground to make up," he wrote. "We need to do something about it."
Messina also referenced the fundraising of Republican super PACs, including those organized and backed, he said, by "the Koch brothers, Sheldon Adelson, Karl Rove, and anonymous billionaires."
Super PACs raise and spend their funds separate from campaigns.
On the campaign trail Friday, Obama gave supporters a preview of the remaining months to the November election, when the campaigns and super PACs on both sides of the aisle are expected to spend heavily from their massive bank accounts.
"Over the next four months, you will be bombarded with more negative ads," he said in Poland, Ohio. "You've got these super PACs, millionaires, billionaires writing $10 million checks, just pouring - raining down on my head."
Someone in the audience shouted, "It's all right."
"It is all right," Obama replied, "I'm skinny, but I'm tough."
– CNN's Mark Preston and Gregory Wallace contributed to this report