Washington (CNN) - The big bucks being raised by Mitt Romney and the pro-Republican independent groups aiding his bid for the White House are a major concern, say top members of President Barack Obama's re-election team.
And one of the officials says he expects Romney's GOP presidential campaign to bring in $100 million in June, out-raising the Obama campaign for a second straight month.
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"I think he's going to have a $100 million month this month, between him and the [Republican National Committee]," said the official, who, along with other top members of the re-election team, spoke to reporters on background Wednesday. "I think Romney's going to continue to have big months. Combine that with the super PAC stuff and we're going to be the first incumbent outspent. That's clear."
Romney and the RNC out-raised the president and the Democratic National Committee by more than $16 million last month. And independent pro-Republican groups such as Crossroads (co-founded by Karl Rove), Americans for Prosperity (funded by the Koch brothers), and Restore Our Future (the super PAC dedicated to electing Romney president), have outraised and outspent the pro-Democratic groups that support the president's re-election.
Obama campaign officials predict a total of $1.225 billion will be raised by Romney's campaign, the RNC, and pro-Romney independent groups. And that worries top re-election officials.
"The two things that concern me about this race are externalities that we can't control. One is super PAC spending," says one of the officials. "The fact that Romney and his assorted super PACs are spending $20 million in 10 days in the last couple weeks in June is a harbinger of what's to come."
"It's a source of concern and there's no point in being cute about it," added the official. "We're having to use our money to respond to super PAC attacks and that puts us at a disadvantage that Gov. Romney doesn't have in this race."
The officials also highlighted that much of the money raised on the Republican side goes to independent groups that legally don't have to disclose their donors.
"Three quarters of the money that's being deployed right now is undisclosed money. You have no idea who's writing those checks," said the official. "What is it that they're trying to hide from the American people?"
While super PACs report to the Federal Election Commission and are required by law to publicly reveal their donors, groups such as Crossroads GPS, which has gone up with tens of millions of dollars of anti-Obama ads in key battleground states, report to the IRS and are set up as non-profit 501(c)(4) organizations do not have to reveal their contributors.
On Tuesday, Obama's re-election campaign filed a complaint claiming Crossroads GPS is in violation of election law by not disclosing its donors. In a letter to the Federal Election Commission, a lawyer for Obama's campaign argued that Crossroads GPS is a political committee–not a charitable organization–and therefore should meet disclosure requirements.
"We're going to push through in the courts, if necessary, to get exposure," said the official, discussing their legal push. "We are going to try and make clear where this money is coming from, what the motivation of those donors are who are writing the checks."
Responding to the complaint, Crossroads spokesman Jonathan Collegio described it as a "goofy sideshow" and pointed to a group supportive of the president.
"Folks would do well to consider this a goofy sideshow until Obama sends the same letter to Priorities USA - the group modeled after Crossroads but which supports the president. Obama doesn't care about the campaign laws; he only cares about silencing conservative groups that are holding him accountable for his failed record," Collegio said.
Priorities USA, the pro-Obama group, is also a 501(c)(4) non-profit group that is not required to disclose its donors. Similarly, it's connected to a super PAC, Priorities USA Action, to which the Obama campaign has urged donors to contribute.
Regardless of their concerns, the re-election team is confident they can overcome their expected disadvantage on campaign cash.
"I think we have certain advantages that will allow us to overcome that. The president is a well known person in this country, he's got a big following. We've got a big organization. While we don't have $10 million donors, we do have millions of donors who are giving an average of $51 a piece, and will chip away at it," said one official.
"I think we'll have the resources to do what we think we need to do to win this election," added another official.