Among some of the contentious moments of the debate, the candidates sounded off on the Hunter Biden controversy, impeachment proceedings and income inequality.

Former Congressman Beto O'Rourke of Texas spoke more at Tuesday's debate than at previous debates, although part of his time was spent in a testy exchange with Indiana Mayor Pete Buttigieg on gun buybacks.

Elizabeth Warren was called out by rivals for a change, Joe Biden failed to stand out and Bernie Sanders calmed concerns about his health. Pete Buttigieg had his biggest night so far.

Pete Buttigieg: The South Bend mayor had one clear goal in the debate: Hit Elizabeth Warren on her support for "Medicare for All," and make sure Democratic voters knew he had an alternate plan that would not eliminate the private health insurance market. Mission accomplished. And remember: The polling I've seen makes clear that voters prefer a plan that preserves the right to choose a private insurance plan than one that gets rid of the private market in favor of a government-run plan. Buttigieg didn't stop there. His response to Rep. Tulsi Gabbard's call to end "endless wars" was powerful, leaning heavily on his own military service. He slammed former Rep. Beto O'Rourke on gun control, with one of the lines of the night: "I don't need lessons from you in courage, political or personal." From beginning to end, Buttigieg was a dominant and commanding force. Yes, some will say he was "mean." But debates -- and primaries! -- are about drawing contrasts, and that is what Buttigieg did. And did very well.

Nothing Harris did on Tuesday night will change her trajectory which is downward in this race. Her "Dude gotta go" line about Trump fell flat, as if the audience had sort of been there and done that. Harris' attempt to force Warren to agree with her that Donald Trump's Twitter account should be suspended felt small and not terribly effective. Harris has simply not been able to recapture the magic she had in that first debate of this election; Tuesday night was another swing and miss.

Meanwhile a note about Tom Steyer: When the most interesting thing about you in the debate is that you wore a plaid Christmas tie, you didn't have a good debate.