Half a million people are expected to vote in New Hampshire on Tuesday.
Trump's campaign is trying to moderate his image and showing a different side of the boastful billionaire.
The Ohio governor has built a base of support among moderate Republicans and independent voters.
The former president escalated his attacks on the Vermont senator over the weekend.
Unlike other debates, ABC News will not hold an undercard for low-polling candidates.
Trump's lead in New Hampshire has narrowed since Iowa.
In the last debate before the New Hampshire primary, Clinton came prepared for Sanders's critiques.
The Washington Post reported that Carson planned to eliminate 50 staff positions, or nearly one-half his staff.
In a Wednesday email, the candidate accused the media of spreading "false" and "misleading" information.
Trump this week has switched his rhetoric from blaming his second-place win in Iowa on his recent decisions, to attacking Ted Cruz for "fraud" in the state.
The Granite State's first-in-the-nation primary is less than a week away.
This week was not the first time the governor was faced with the decision to continue with his personal plans out of state or return home.
The Ohio governor tied with Ted Cruz for second place in a new WBUR poll in the early-voting state of New Hampshire.
The three Democratic candidates will have a chance to appear before Iowa voters a week before the state's caucuses.
Sanders is beating Clinton 60-33 in New Hampshire, a new CNN/WMUR poll shows.
In a lighthearted interview with comedian Jimmy Fallon, Trump said Clinton's email scandal is what's keeping her from clearing the field of Sanders.
Trump said he influenced the network's decision to cut the New Hampshire paper from the debate.
Clinton spoke about his personal relationship with his wife while campaigning for her in New Hampshire.