On Thursday Bernie Sanders had a meeting with President Obama at the White House. Afterwards he gave a short speech outside the West Wing. Of course Senator Sanders displayed many body language signals - and one in particular is highlighted here.
This near microexpression was captured during 4:43 the instant after the Vermont Senator says, "... I spoke briefly to Secretary Clinton on Tuesday night ..."
In this close up we can more easily see clearly the primary emotion being displayed here: that of bitter-regret (while there is a secondary component of anger)
This same facial configuration is also displayed at the very end of the video (during 5:05) after Senator Sanders finishes his speech is turning away.
He knows this signals the (very near) end of his campaign and thus it's very natural and expected for him to feel a deep and bitter-regret.
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