Republican presidential candidates Marco Rubio and John Kasich have suggested they may not support Donald Trump if he becomes the Republican nominee, as violence at the front-runner's rallies deepened the party's chaotic chasm.

Newt Gingrich says Mitt Romney is the weakest government opposition party front-runner since 1920, but his sizeable campaign fund allows him to run well in races where his rivals don’t have the resources to campaign.

Demonstrations near the Republican National Convention site turned violent as protesters harassed delegates, smashed windows, slashed car tyres and hurled bottles.

The war for the White House is being fought on many fronts, and among the battles is that between the Bush twins and the Kerry daughters.