India, Nov. 7 -- A Democratic takeover of both chambers of Congress weakens Donald Trump's agenda, but he remains in office with substantial executive powers. His ability to pass legislation shrinks, while Democratic lawmakers gain new ways to investigate and challenge his administration.

The election's political message depends on more than party totals. A change in control could reflect concerns about the economy, immigration or Trump's leadership, but the results alone do not establish why voters made their choices.

A Democratic sweep means a serious rebuke to a president who has closely tied Republican politics to his own leadership. Individual candidates and local conditions matter, but control of both chambers would be read as a n...