Mitt Romney's campaign team has appealed for his rivals to quit the Republican presidential race, claiming that it is near impossible for them to catch him after the Super Tuesday results and that by staying in they are boosting President Barack Obama's re-election chances.

But Rick Santorum, boosted by his three wins on Tuesday and running Romney close in Ohio, was out on the campaign trail again on Wednesday and stubbornly refusing to concede, intent on pushing Romney to the end. The former House Speaker, Newt Gingrich, too was out campaigning and refusing to pull out.

Another series of bruising and bitter contests beckons, with the next big test in Illinois on 20 March, which will again pit Romney against Santorum and expose Republican faultlines.