Rep. Kyrsten Sinema (D-Ariz.) talks to campaign volunteers at a Phoenix campaign office on the day of the primary election, Aug. 28. (Ross D. Franklin/AP)

Wealthy donors — including a former GOP backer — injected millions last month into the main Democratic Senate super PAC, a sign of intensifying hopes on the left of flipping control of the chamber.

Senate Majority PAC, which works to elect Democrats to the chamber, posted a strong haul in August at $17.6 million, far above its monthly fundraising average of $10.5 million for the midterms. The super PAC has raised $94.4 million so far, compared with $61.5 million by its GOP counterpart, according to new Federal Election Commission records posted Thursday evening.

Among the biggest donors to the PAC last month was Seth Klarman, a Boston hedge fund billionaire and registered independent who was once the biggest GOP donor in New England.

Klarman is almost exclusively giving to Democrats this election in an effort to help them regain control of Congress — which he has said he views as a necessary step to hold President Trump accountable.

Last month, Klarman gave $1 million to the Senate Majority PAC and $2 million to the super PAC supporting House Democrats, FEC records show.

Klarman, chief executive of the Baupost Group, has pledged to spend up to $20 million to support Democrats this fall — which would place him squarely among the 2018 election’s biggest donors to super PACs, alongside Democratic heavyweights such as Tom Steyer and George Soros. He also gave the maximum allowed contribution to the Democratic National Committee.

In comparison, Klarman gave $2.9 million to super PACs in 2016, largely to support Republicans, a Washington Post analysis shows.

Klarman, who rarely speaks to the media, recently told a New York Times columnist that he felt “betrayed” by “spineless” Republicans, saying GOP leaders in Washington have largely been “profiles in cowardice.”

“We need to turn the House and Senate as a check on Donald Trump and his runaway presidency,” Klarman told the Times.

A spokeswoman for Baupost on Thursday pointed to Klarman’s interview with the Times and declined to provide further comment.

Other wealthy donors spent hefty sums last month in support of Democrats running for the Senate.

Republicans hold a thin majority of 51 seats in the upper chamber but are generally considered to have a better shot this midterm election at maintaining the majority than in the House. Democrats must pick up two Senate seats and defend 26 others, including five that are in states Trump carried by double digits.

Among the donors who spent heavily in August to support Democrats were Soros, a hedge fund founder and major Democratic donor, and Fred Eychaner, a media mogul and head of Newsweb Corp., the new filings show.

Eychaner gave $4 million to the Senate Majority PAC, and Soros gave $1.4 million in August, records show. Eychaner gave an additional $2 million last month to House Majority PAC, the super PAC supporting House Democrats.

The Senate Leadership Fund, the super PAC supporting Senate Republicans, raised $3.2 million in August. Among its biggest August donations were $1 million from Tennessee-based petroleum company Pilot Corp. and $500,000 from Koch Industries.

In addition, new filings posted Thursday revealed the donors to Red and Gold, a super PAC that formed just before the Senate primary race in Arizona, home to one of the most competitive contests this fall.

Because the super PAC organized so close to the primary election, it was not required to disclose its donors until the end of the month — after the election was over. The super PAC raised $2.9 million in August and spent more than half of it on ads attacking Martha McSally, the front-runner who won the GOP nomination.

Red and Gold reported receiving $1.7 million from the Senate Majority PAC. It also received $600,000 from Soros and $500,000 from James Simons, another major Democratic donor and hedge fund manager.

The Republican National Committee again posted strong numbers for August, consistent with its massive fundraising haul and war chest so far this cycle. The RNC raised $16 million last month, compared with the Democratic National Committee’s $9.2 million.

In all, the RNC has raised $231 million this election cycle — nearly double the $122.2 million the DNC has raised. The RNC had $41.8 million in cash by the end of August and the DNC had $8 million on hand, with $7 million in debt.