Kamala Harris is ripping Donald Trump as an unserious man ... but she says the threat he poses to America is very real ... and she says he's out of his mind.

The Vice President blasted Trump in accepting the Democratic nomination for president Thursday on the final night of the DNC at the United Center in Chicago.

Harris was coasting through her speech when she finally turned her sights on her November opponent ... warning a second Trump presidency would be devastating to Americans, particularly the middle class.

The Veep threw a flurry of punches at Trump ... raking him over the coals for the Jan. 6 Capitol riots and painting him as a man who only cares about himself and no one else.

Harris cited SCOTUS giving the former president protection against prosecution and warned ... imagine Trump in the White House with no guard rails.

The Democratic nominee also ripped Trump for sabotaging a bipartisan border security bill ... vowing to revive it and sign it into law if elected.

Harris said she would be a president for all Americans, regardless of party ... and warned Trump would "serve the only client he has ever had -- himself."

President Biden's VP even compared Trump to ruthless dictators ... and said Russia and North Korea are rooting for him to win the election.

Something tells us Trump's social media response is going to be pretty charged too.