Barack Obama has failed to deliver on his promise of hope and change and it is time for new leadership in the White House, Mitt Romney told America today, in the most critical speech in his presidential campaign.

Mitt Romney will step up for the most important speech of his Republican US presidential campaign tonight, after a fiery warm-up from a running mate who said President Barack Obama was “grasping at a moment that has already passed.”

Mitt Romney, freed of his last serious challenger for the Republican presidential nomination, has unleashed blistering attacks on Barack Obama, assuring a brutal tone for the coming election campaign.

Barreling out of Florida with money and momentum on his side, Republican presidential front-runner Mitt Romney said yesterday that the bare-knuckled nomination fight thus far has toughened him up for contests to come.

Team Obama has quietly built a juggernaut re-election machine in Chicago. Andrew Romano goes inside