Billionaire liberal activist George Soros and his son helped underwrite the group that launched a barrage of negative ads attacking Republican Senate hopeful Martha McSally during the final stages of her competitive GOP primary.

Newly filed campaign finance reports finally make clear who was behind the group Red and Gold and its $1.7 million in spending against McSally, who still handily won the Republican nomination.

McSally was widely viewed as more electable than her GOP rivals Kelli Ward and Joe Arpaio and now faces Democrat Kyrsten Sinema in the Nov. 6 general election.

From the group's curious name to its previously secret donors and its decision to aggressively target the GOP front-runner even before she secured the nomination, Red and Gold made a memorable splash in August.

But the group has been silent in the weeks since the primary.

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The group, whose spokesman Rodd McLeod frequently blasts Republicans for refusing to be transparent about the political financiers backing their candidacies, would not disclose to The Arizona Republic the source of Red and Gold's funding. He said reporters, and the public had to wait until the group was required to report the information with the Federal Election Commission.

"We've always said we're a group of Democrats trying to elect Kyrsten Sinema," he said Friday. "We've never tried to pull the wool over anyone's eyes about it."

He compared that transparency to McSally's public comments saying she wanted to preserve insurance coverage for those with pre-existing conditions, then voting repeatedly to take that away under the GOP health care bills last year.

Within days of forming, Red and Gold received $1.7 million to attack McSally in the primary from Senate Majority PAC, a political action committee aligned with Senate Minority Leader Chuck Schumer, D-New York.

That group's major donors include Alexander Soros, George Soros' son.

George Soros provided $600,000 to Red and Gold two days after the Aug. 28 primary. That's in addition to the $425,000 in anti-McSally spending by Priorities USA Action, another group he helps fund.

James Simons, a prolific Democratic donor who co-founded Renaissance Technologies, a hedge fund, gave $500,000 to Red and Gold on Aug. 14.

Simons is a renowned mathematician and investor, but is also known for his efforts at avoiding billions in taxation.

The only other individual donor for Red and Gold was Austin Marxe, the CEO of AWM Investment, who gave the group $100,000.

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For Senate Majority PAC, also known as SMP, the stealth efforts to tip the GOP primary echoes its more-successful — and initially secret — spending in Alabama's special Senate election in December.

In that race, Democrat Doug Jones defeated Republican Roy Moore, whose campaign collapsed after a series of allegations he made sexual advances on teen girls when he was in his thirties.

SMP's involvement also reflects a measure of political pragmatism for Schumer.

Sinema has said she would not back him for Senate majority leader if she is elected to the upper chamber.

That kind of rebellion is nothing new for Sinema, who has cast votes for Democratic U.S. Rep. John Lewis of Georgia to be House speaker rather than U.S. Minority Leader Nancy Pelosi, the California Democrat who is villainized on the right as a symbol of liberal politics in Washington.

Jon Seaton, a Republican strategist, ridiculed Red and Gold's efforts after its donors became clear Thursday.

"We set $2.3 million on fire and all we got was the one person we didn't want to face in November," he wrote in a tweet he mockingly attributed to Schumer and Soros.

Republic reporter Yvonne Wingett Sanchez contributed to this report.

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