Senate Republicans fear Elon Musk’s beef with President Trump could become a wild-card factor that could cost them seats in 2026 if Musk follows through on his threat to create a new political party to compete with the GOP. GOP lawmakers note that third-party candidates have swung presidential and Senate races in the past and worry that Musk’s bid to establish an “America Party” is likely to peel off more Republicans than Democratic voters in key races. Musk, if he moves forward with his plans, is expected to make reducing the federal debt a key plank of his new party,...