The bitter fight between Donald Trump and Hillary Clinton continues to heighten.

Donald Trump speaks at a campaign rally on December 21, 2015 in Grand Rapids, Michigan. (Photo by Scott Olson/Getty Images)

The two party front-runners are in the midst of an escalating battle.

Trump called Clinton's debate bathroom break 'Disgusting' and used strong words to bash her 2008 loss to President Obama in the primary.

Clinton responded by saying she doesn't believe Trump has any boundaries at all and that bullying has become his campaign.

Engaging with Trump demonstrates a growing confidence for Clinton in the primary against Bernie Sanders.