WASHINGTON — Another 2.6 million people slipped into poverty in the United States last year, the Census Bureau reported Tuesday, and the number of Americans living below the official poverty line, 46.2 million people, was the highest number in the 52 years the bureau has been publishing figures on it.

And in new signs of distress among the middle class, median household incomes fell last year to levels last seen in 1996.

Economists pointed to a telling statistic: It was the first time since the Great Depression that median household income, adjusted for inflation, had not risen over such a long period, said Lawrence Katz, an economics professor at Harvard.

‘This is truly a lost decade,