The Democratic and Republican presidential contenders have begun defining their approach to major foreign policy issues as they jockey for position in their parties’ primaries.

The president-elect knows only too well the failures of all the presidents who preceded him, and the hardening of positions that's occurred on both sides.

Fifty years ago, Republicans turned on President Richard Nixon. Today, most of the party continues to stand by Trump. Why the difference? A rise in partisanship.