Elon Musk is putting a small fortune into Republicans’ effort to salvage the Senate seat in Texas, seen as critical to the party keeping control of the chamber in the fall elections.

According to federal filings, Musk donated $158 million to Senate Republicans’ super PAC in September—the same amount that was immediately funneled to a group backing Republican Ken Paxton’s Senate bid.

The new disclosures show that Musk, the world’s richest person, is behind much of the money that has been pumped into the Texas Senate race to help keep Paxton financially competitive as his Democratic opponent James Talarico has significantly outraised him throughout the race.

Musk is a Texas resident and has several companies, including Tesla and SpaceX, based in the Lone Star State.

In September, Musk sent two large donations to the Senate Leadership Fund, Senate Republicans’ main super PAC.

On Sept. 9, the first day of the Republican midterm convention in Dallas, Musk funneled $75 million to SLF. The following day, the super PAC moved $75 million into a connected entity named Texas PAC. Two weeks later, Musk gave $83 million to SLF—and again, the following day, the group sent the same amount to Texas PAC, which is the biggest advertiser in the Texas contest, according to AdImpact data.

And in August, he also gave $10 million to Lone Star Liberty PAC, a super PAC boosting Paxton.

Overall, Musk gave more than $250 million to Republican-aligned groups in the third quarter, campaign finance filings show. Among that was $89 million he gave to his own America PAC.

“If Democrats take control of Washington and win key gubernatorial and attorney general races this November, then America risks becoming a socialist, woke and borderless country,” said Andrew Romeo, a spokesman for America PAC. He said the Musk donations are designed to give Republicans “a fighting chance to defy history.”Musk launched America PAC to support Trump’s 2024 election, and the group now backs Republicans in competitive races across the country, spending over $23 million since July to back individual candidates.

The GOP has a 53-47 majority in the Senate. Democrats are feeling increasingly bullish about their chances of flipping the Senate, in part due to their strong showing so far in Texas.

Talarico has consistently held a narrow lead in polls, raising Democrats’ hopes that they could win statewide office in Texas for the first time since 1994. Such hopes for the party have repeatedly fallen short, most notably when former Rep. Beto O’Rourke narrowly lost to Sen. Ted Cruz in 2018.

The matchup between Paxton, the state attorney general, and Talarico, a state lawmaker, is one of the most expensive races this election cycle. And Republicans are dramatically outspending Democrats in the final stretch of the race.

For the individual campaigns, Talarico raised $64 million in the third quarter this year, his campaign said—more than three times the over $18 million Paxton reported in the same quarter.

Going into the midterms, Republicans had a money advantage over Democrats. They have been throwing hundreds of millions of dollars into races across the country. Still, polls haven’t shown significant shifts in high-profile contests since more money started pumping in.

Write to Sabrina Rodriguez at sabrina.rodriguez@wsj.com and Anthony DeBarros at anthony.debarros@wsj.com