WASHINGTON — Mitt Romney’s presidential campaign has raised at least $100 million in August for the third consecutive month, building upon a fundraising prowess that has allowed him to outraise President Barack Obama so far this summer.

The early numbers, which include money raised by the national Republican Party, will be publicly released next week.

The numbers were revealed today, the first day of the Democratic National Convention in Charlotte, N.C., where delegates will nominate Obama for a second term. Federal records show that Romney and the GOP have pulled in more cash than Obama’s re-election effort in May, June and July, including money collected by the Democratic Party.

It is usually difficult for a challenger to raise more money than an incumbent holding a major elective office, particularly in a presidential race.

The figures exclude tens of millions of dollars that outside “super” political action committees are pouring into the race to help Romney.

While once a record-breaker — Obama raised a remarkable $750 million four years ago, the president’s advisers are publicly acknowledging the incumbent likely will be outspent.

Obama and the Democratic Party have not released their August fundraising figures.