Two former Trump allies jump into the race to challenge him

<p>As the GOP primary field expands, its latest entries this week are marked by the candidacies of former Vice President Mike Pence and former Gov. Chris Christie (R-N.J.) — the head of the Trump White House transition team and formerly thought of as a top VP candidate. Amy Walter, publisher and editor-in-chief of the Cook Political Report, and Michael Kruse, senior staff writer for Politico, join Chuck to size up the developing primary.</p>