CHARLOTTE, N.C. — President Trump inserted himself on Monday into the Democratic presidential nomination contest on the eve of its biggest day, belittling the resurgent Joseph R. Biden Jr. as a feebleminded has-been while fanning resentment among supporters of Bernie Sanders.

Traveling to one of the larger states holding a Democratic primary on Super Tuesday, Mr. Trump eagerly sought to undercut Mr. Biden at the very moment the former vice president revived his flagging campaign, taunting him for verbal gaffes and memory lapses and portraying him as an old man who even if elected would not govern but instead be committed to a retirement home.

As he has for weeks, the president likewise sought to encourage disenchantment on the left by promoting the notion that the Democratic Party establishment was trying to somehow illegitimately take away the nomination from Mr. Sanders. But Mr. Trump did not neglect the rest of the shrinking Democratic field, firing off barbs at the other candidates still in contention and even those who dropped out in the past 48 hours and threw their support to Mr. Biden.

“The Democrat party has gone crazy,” Mr. Trump said to a stadium full of cheering supporters.

The president wrapped up his rally not long before Mr. Biden’s own election-eve rally in Dallas got underway, and Mr. Trump seemed especially focused on the former vice president as a potential opponent in the fall.