Romney and the Republicans raised $106m (£68m), $35m more than Obama and the Democrats, who raised $71m.
Both camps raised more than in May, when the Romney bid took in $77m and the Obama camp $60m.
The totals exclude millions raised by independent groups that support each candidate.
The Obama campaign released its June numbers in an email to donors, reports the BBC.
"If we lose this election, it will be because we didn't close the gap enough when we had the chance," the email says, issuing another call for donations.
Obama's campaign has been regularly warning supporters that he is in danger of becoming the first sitting president in history to be outspent by his opponent.
Romney and the Republican National Committee have an estimated $160m cash in the bank to spend on the campaign, they confirmed on Monday.
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