This week’s Capitol Hill votes affirming the legitimacy of Donald Trump’s war in Iran may have buttressed the President’s authority to extend executive prerogatives to declare war. But they also underlined the fragility of his grip on power in Washington.

Mr. Trump prevailed in the Senate on Wednesday, which failed by a 47-to-53 margin to move toward restricting his power to undertake military action. The vote in the House of Representatives Thursday supporting his conduct in the war on Iran was 219 to 212.

Together the twin votes were less a demonstration of the power of Mr. Trump’s sway in American politics than a vivid display of the potential limits on it. The Republican majorities in both houses of Congress are slim, the midterm congressional elections are eight months away, and polls suggest a decisive shift away from the GOP. The loss of Republican control in either house likely would severely restrict the range and extent of Mr. Trump’s power − and would deliver to Democrats subpoena power they could use to torment administration officials and, in unlikely extreme cases, remove them.