Roughly 37.2 million Americans tuned in for Mr. Trump’s speech, Nielsen said on Wednesday, the smallest live-TV audience for a presidential address to Congress since Barack Obama’s valedictory State of the Union in 2016.

The viewership on Tuesday was a 20 percent decline from Mr. Trump’s State of the Union last year, and well below the audience for his speeches in 2017 and 2018.

Why the dip? For one thing, viewers may be feeling a bit overwhelmed.

A Pew survey last fall found that 66 percent of Americans said they were “worn out by the amount of news these days.” And those numbers predated Mr. Trump’s impeachment, American military conflict in Iran, the president’s trial in the Senate and this week’s Iowa caucuses — the outcome of which, as of Wednesday evening, was still in limbo.