Republican strategist Karl Rove spoke at the El Paso County Lincoln Day Dinner Wednesday night, June 1, 2011 at the Antler's Hilton Hotel in Colorado Springs. Karl Gehring/The Denver Post (Photo By Karl Gehring/The Denver Post via Getty Images)

WASHINGTON -- Billionaires are rushing to the aid of Republicans in the homestretch of the 2014 elections with a massive super PAC infusion after a plea from big money mastermind Karl Rove.

On Sept. 17, Rove, the co-founder of the super PAC and dark money duo American Crossroads and Crossroads GPS, took to the Wall Street Journal editorial page to send out his billionaire bat-signal.

If the rich want to ensure Republican control of the Senate, "reducing the Democratic cash advantage will tip the needle in the GOP's direction," Rove wrote. "That will only happen if Republicans open their wallets to candidates whom they may have never met."

Since Rove's plea, 21 billionaires and their family members have poured $19.9 million into super PACs backing Republican Party candidates, fueling an October outside spending surge. Only $5.1 million was given to the Republican super PACs in the first half of September, before Rove's request.

The billionaire cash infusion came from what has become the core of the party's disclosed donors ever since the 2013 deaths of Houston-based homebuilder Bob Perry and the Dallas investor Harold Simmons. This central network includes hedge fund billionaires Paul Singer, Ken Griffin, Julian Robertson, Daniel Loeb and Seth Klarman, coal mining executive Joe Craft, World Wrestling Entertainment co-owner Linda McMahon, TD Ameritrade founder and Chicago Cubs owner Joe Ricketts, investors Robert Rowling and B. Wayne Hughes, and Houston Texans owner Robert McNair.

Billionaire brothers Charles and David Koch, known predominantly for their secret funding of conservative political causes, gave $2 million each to their Freedom Partners Action Fund over the summer.

In September, Rove's American Crossroads and Ricketts' Ending Spending Action Fund were the top two recipients of billionaire contributions with a combined $14.3 million.

A host of super PACs supporting just one candidate also received big contributions in the month. Groups formed with the sole purpose of supporting GOP Senate candidates Scott Brown in New Hampshire, Rep. Tom Cotton in Arkansas, Joni Ernst in Iowa, Dan Sullivan in Alaska, Mitch McConnell in Kentucky and Thom Tillis in North Carolina each received at least $500,000 from McNair, along with contributions from other billionaires.

This infusion has finally pushed spending by pro-Republican nonparty groups past the totals reported to the Federal Election Commission by similar Democratic Party groups.

For the entire 2014 election cycle leading up to September, pro-Democratic groups reported spending more than $65 million in support of their candidates while Republican groups reported expenditures of $51 million.

For September, Republican Party groups reported spending $52 million to the Democrats' $41 million. The first half of October has seen continued Republican gains with $71 million doled out by these groups compared to just $39 million for Democrats.

Top Republican-Aligned Groups Gain Upper Hand In Final Months of 2014 Elections

Source: Federal Election Commission. Additional note: Totals are based solely on disclosures made to the FEC. This does not include "issue ad" spending by nonprofit groups before September 2014. The totals also do not include spending by groups within contested party primaries.

The numbers reported to the FEC, however, greatly underestimate the support candidates had earlier in the election cycle from affiliated nonprofit groups. The organizations, which mostly support Republicans, are largely used to shield the identity of donors and do so by running "issue ads" that promote or attack given candidates while stopping short of calling for their election or defeat.

These dark money groups spent at least $64 million on unreported issue ads aiding Republicans. Similar groups on the Democratic Party side spent just $24 million.

What is new in the late rise in pro-Republican spending is the resurgence of the party's super PACs. In 2012, super PACs -- independent groups that can raise unlimited sums from corporations, unions and individuals, but must disclose their donors -- were major players in both the presidential and congressional races, particularly for Republican candidates. The party's failure to capture the White House or control of the Senate led many donors to shun these groups.

Super PAC spending by Republican groups totaled just $46 million at the end of September. Democratic super PACs spent $87 million.

Thanks to Rove's unleashed billionaires, this dynamic has flipped. Republican super PAC spending surged above $37 million through the first half of October, while Democratic groups spent just $28 million. In the same time period, Republican dark money groups spent more than $23 million -- more than five times the amount spent by those types of groups on the Democratic side.

In the end, this late-stage Republican outside spending fueled by billionaire donations may be a little too late. Many of the Democratic groups reserved television and online advertising time earlier in the cycle at lower rates. Now that there are a crush of buyers, ad rates aresoaring. The billionaires may have come in time to pump up Republican spending, but GOP groups aren't able to buy as many ads as Democrats have.

The late spending binge does underscore Democratic Party fears of the world of unlimited money unleashed following the Supreme Court's 2010 Citizens United decision. For Democrats, there simply are not enough billionaires willing to write excessively large checks to support the party.

Where Republican super PACs received $25 million from 21 billionaires in September, Democrats were given a little more than $20 million from just 11 billionaires. Nearly all of this -- $15 million -- came from Tom Steyer.

A hedge fund investor turned environmentalist, Steyer has spent the most in the 2014 election cycle, with more than $57 million in contributions, according to disclosed super PAC records. These huge contributions put him in a league of his own among Democratic billionaires and allow him to tilt spending toward his personal priorities, which have not entirely aligned with the party's this cycle.

Of the few billionaires who do give to Democratic super PACs, many are not entirely team players.

Billionaire and former New York Mayor Michael Bloomberg gave $2.5 million to super PACs in September. While often categorized as a supporter of the Democratic Party due to both his contributions and his support for gun control and believing in climate change, Bloomberg's actual spending tells a mixed story.

The former three-term mayor has given a total of $14.6 million to super PACs in the 2014 election cycle including $7.7 million to his own Independence USA PAC. Democratic super PACs have received $4.75 million from Bloomberg, but he has also given $500,000 to groups supporting Sens. Thad Cochran (R-Miss.) and Lindsey Graham (R-S.C.), respectively.

Further, Bloomberg's Independence USA PAC has spent only $732,000 on a non-primary election supporting a Democrat, Sen. Cory Booker (N.J.). (The super PAC also spent $2.1 million in a Democratic primary special election to support the pro-gun-control candidate Robin Kelly, who is now representing Illinois' 2nd Congressional District.)

On Monday, though, Bloomberg's super PAC announced $1.9 million in expenditures to support the comeback bid of former Rep. Bob Dold (R) in Illinois.

Billionaire energy investor John Arnold and his wife, Laura, gave $190,000 to a super PAC backing California Democratic House candidate Ro Khanna. Khanna's opponent, however, is not a Republican; instead it's labor-friendly incumbent Rep. Mike Honda (D). Arnold is known as a proponent of cutting public pensions and privatizing public school systems while Khanna has positioned himself as further to the right on economic issues than Honda.

There are still billionaires solely dedicated to helping Democrats in the late stages of 2014. Billionaire investor George Soros gave $1.25 million to super PACs in September. Laurene Powell Jobs, widow to Apple's Steve Jobs, gave $500,000, her six-figure super PAC contribution, and Hollywood director Steven Spielberg gave $250,000 to Senate Majority PAC.