WASHINGTON—In laying out the checks and balances of democracy, authors of the U.S. Constitution imagined that the president and Congress would wrestle for power when their authority conflicted, each fighting for influence. “Ambition must be made to counteract ambition,” James Madison wrote. President Trump’s ambition has been sweeping and muscular in the first days of his new term, producing less of a transition in government than a takeover. Congress, by contrast, has all but forfeited the match. The result, if Trump’s assertions of power survive the courts, would significantly rebalance power in Washington, centralizing unprecedented authority over federal spending, executive-branch...