It was a dire warning, delivered on the biggest stage: Some 278,000 noncitizens were improperly registered to vote in federal elections and were poised to tilt the scales of the election in November, President Donald Trump warned in a primetime address to the nation in July.

"The real number is actually much higher," he added.

The president's bold claim set off a series of events that culminated in recent weeks with an unprecedented bid by the federal government to impose itself on state-run elections.

The Trump administration has sued the state of California and threatened other states over the issue of noncitizen voting, a longtime concern of the president and his administration despite experts saying it is already illegal and " vanishingly rare ." Federal officials have demanded action after presenting to states what they say are lists of noncitizens who are registered to vote, gathered using a combination of federal immigration data and commercially available state voter registration rolls.

But in…