Mitt Romney’s cash advantage over President Obama and the Democrats more than doubled in July, as intense Republican fund-raising and heavy spending by Mr. Obama and his allies left Mr. Romney and the Republican National Committee with $62 million more in the bank than the Democrats at the end of last month.

Mr. Obama’s campaign and the Democratic National Committee spent $91 million in July, significantly more than the $75 million the Democrats raised, underscoring the investments Mr. Obama made in technology and field staff as well as nearly $40 million his campaign spent on advertising that month. While Mr. Romney continued to husband his resources for the fall — he spent less than half of what Mr. Obama did on advertising — conservative “super PACs” and other outside groups stepped into the breach, spending millions of dollars on ads attacking Mr. Obama.

Mr. Obama and the Democrats raised a combined $75 million during July, according to Federal Election Commission reports filed Monday, compared with $101 million for Mr. Romney and the Republicans. The Democrats had about $124 million in cash on hand, most of it in Mr. Obama’s campaign account, while the Republicans had $186 million.

It is not unusual for challengers to outraise incumbents late in the campaign, and while Mr. Obama is being outraised now, he raised far more than Mr. Romney through 2011 and early this year.