WASHINGTON—With a 4:15 a.m. ET social-media post on Wednesday, Elon Musk declared that a must-do spending bill “should not pass.” By early evening, the bill was dead, leaving the government barreling toward a weekend shutdown just before Christmas. Lawmakers who might have underestimated Musk’s ability to shake up Washington were suddenly having second thoughts.

Over the course of Wednesday, Musk pressed for Congress to kill the bill. He encouraged his more than 200 million followers on X, the social-media platform that he owns, to call their representatives to vote against it, and he warned that Republicans who voted for it should lose their congressional seats in two years. He also said Congress shouldn’t pass any more legislation until President-elect Donald Trump takes office, which would ensure a partial government shutdown until Jan. 20.