A flood of last-minute largesse from Democratic donors helped Priorities USA Action, the super PAC backing President Barack Obama's reelection, outraise its Republican counterpart for the second month in a row in September. But Republican-leaning outside spenders still headed into the last full month of the campaign with a huge cash advantage, an analysis of records filed overnight at the Federal Election Commission by Sunlight's Follow the Unlimited Money tracker shows.

Restore Our Future, the super PAC founded by former Mitt Romney aides to support the Republican presidential candidate, headed into the last month of the campaign with nearly $16.6 million in cash on hand, the largest warchest of any of the outside spenders, according to Follow the Unlimited Money. Restore was closely followed by the Republican super PAC American Crossroads, which has spent $55 million opposing Obama so far. American Crossroads had nearly $15.8 million in the bank at the beginning of this month.

An eye-popping $11.5 million donation from Chicago Cubs owner Joe Ricketts, who earlier had to apologize when word leaked that he was considering a racially inflammatory ad campaign against Obama, helped push the Ending Spending Action Fund into the top five committees that headed into the last month of the campaign with the most month to spend.  Here's the complete list.

While Obama's campaign has ballyhooed the number of small donors it has attracted, the super PAC supporting him had a banner month fueled by writers of seven-figure checks. Priorities USA Action reported more than $15 million in receipts for September (compared to $14 million for Restore Our Future), mostly from reliable Democratic mega-donors. Fred Eychaner of the Newsweb Corp. in the president's home town of Chicago, gave $2 million (a portion of a total of $4.5 million he gave to Democratic super PACs last month). Hedge fund manager James Simons gave $1.5 million (along with another $2 million to other Democratic super PACs in September). Slim Fast founder S. Daniel Abraham gave $1.2 million to Priorities; David Boies, the super-attorney who argued unsuccessfully before the Supreme Court that Al Gore won the 2000 election, ponied up $1 million, as did both Hollywood  moguls Jeffrey Katzenberg and Stephen Spielberg.

Unions, traditionally leaders in getting out the Democratic vote, also began moving big bucks into position: The United Auto Workers put $5.3 million into its education fund and and National Education Association transferred $3.3 million to its education fund in September.

But Republican outside spenders still appear to hold a huge cash advantage, especially when expenditures of groups that don't report donors is considered. Sunlight's Follow the Unlimited Money tracker shows that GOP-leaning political nonprofits (groups like Crossroads GPS, the Chamber of Commerce) have outspent their Democratic counterparts (groups like the Sierra Club and Planned Parenthood) by a whopping $115 million to $24 million so far.

That's on top of what appears to be a big GOP advantage among super PACs. The pro-Romney Restore Our Future  has raised by far the most money of any of the groups spawned after the Supreme Court's Citizens United decision. And even though only about half of Restore Our Future's  $111 million haul has gone towards the general election (the group spent $43 million fending off Romney's Republican opponents in the primaries and caucuses), when the other groups spending to oust the president are counted, Romney appears to have a major lead in outside spending.

MORE:

• Outside spending overview
• Outside spending by contest
• Outside spending by partisan affiliation

Ending Spending, for instance, has made its largest expenditures so far — some $4.4 million — opposing the president's reelection. The contribution from Ricketts, founder of TD Ameritrade, will undoubtedly help finance the $10 million ad buy that the group has said it plans to make in the battleground states of Virginia, Ohio, Iowa and Wisconsin. Sunlight's Political Ad Sleuth, which tracks political ad filings, shows that the group has already been active in Wisconsin. 

Reliable Republican donors continued writing big checks to super PACs in September: Texas business moguls Bob Perry and Harold Simmons were responsible for more $6.5 million in donations to Republican groups. Perry, a homebuilder, gave $3.5 million: $2 million to Restore Our Future, $1 million to Club for Growth Action and $500,000 to Independence Virginia PAC.

Simmons' $3 million went to: American Crossroads ($2.5 million) and Restore Our Future ($500,000).