Although they all have professional debate coaches who presumably advise them accurately (or maybe not), Presidential candidates are very often are slow learners (always rationalizing, "I must be pretty good if I've gotten this far!").
Although there are rare exceptions (can you name one?), the vast majority of the time one cardinal rule of a political debate is to never turn toward your opponent when they are speaking. Doing so makes you look like a follower - not a leader. It's a prime example of very public self-subordination.
In the vast majority of all other scenarios, the exact opposite is true - when engaged in healthy conversation, we should point our entire body (feet, hips, torso, face and eyes) toward the other person (An important nuance here is that it should not be directly - rather offset by approximately 15-20 degrees to one side). Doing so conveys respect. However a debate is a highly-contextualized situation, and not only will turning toward your opponent make you appear acquiescing - it will truly engender your neurobiology to this self-fulfilled (defeat) prophecy as well. You might just as well say, "I'm the beta, he's/she's the alpha".
Nonverbal Communication Analysis No. 3416: LeBron James Catches a Golden State Fan "calling" him a Cry Baby
Nonverbal Communication Analysis No. 3339: First Democratic Presidential Debate - Jim Webb's Proud of Making this Enemy
Nonverbal Communication Analysis No. 3405: Republican Debate, Ben Carson and Timidity
Nonverbal Communication Analysis No. 3407: GOP Debate, the National Anthem and Shades of Napoleon
Nonverbal Communication Analysis No. 3371: John Kasich at the Republican Debate
Nonverbal Communication Analysis No. 3410: Bernie Sanders Apologizes to Hillary Clinton for Data Breach at Democratic Debate
Nonverbal Communication Analysis No. 3268: Advice to the Ten Republicans for Thursday Night's Presidential Debate
Nonverbal Communication Analysis No. 2357: Why is this Image so Sexy? Body Language of "Hide & Seek"