Fredreka Schouten and Christopher Schnaars

USA TODAY

WASHINGTON — The super PAC at the forefront of Democrats' efforts to retain their hold on the Senate has accepted nearly half of its contributions from mega donors who have contributed at least $1 million each.

Nearly $19 million — or 47% — of the $40.2 million that has flowed to the Senate Majority PAC this year came in seven-figure chunks, a USA TODAY analysis of new campaign filings shows. By comparison, American Crossroads, one of the most active Republican super PACs, has relied on $1 million donations for 37% of its money this year.

The two groups are among the biggest outside spenders in midterm races.

The Senate Majority PAC has pumped nearly $36 million into Senate contests, according to a tally by the non-partisan Center for Responsive Politics, which tracks political money. American Crossroads has spent $19 million to shape congressional races.

Democrats have cast Republicans as relying heavily on a handful of billionaires to boost their chances of taking the Senate. Senate Majority Leader Harry Reid, D-Nev., has repeatedly denounced conservative billionaires Charles and David Koch from the Senate floor. Reid's allies run the Senate Majority PAC.

Republicans must pick up six seats to seize the Senate majority.

Senate Majority's $1 million donors last month included Ian Cumming, chairman of Leucadia National Corporation, a company whose holdings include investment banking, beef processing and manufacturing interests.

Another $1 million flowed from HFNWA LLC, a limited liability corporation based in Fayetteville, Ark. Arkansas Secretary of State filings available online show the firm is connected to Frank Haney, a Washington real-estate developer.

Unions and big names in Hollywood and Democratic politics also have come the super PAC's aid in recent weeks. Dreamworks CEO Jeffrey Katzenberg and director Steven Spielberg each gave $250,000 to Senate Majority on Sept. 8.

Financier George Soros and Laurene Powell Jobs, the widow of Apple co-founder Steve Jobs, each donated $500,000 last month.