Presidential candidate John McCain urged sceptical conservatives on Friday to rally behind him and said he might clinch the Republican nomination in crucial 'Super Tuesday' coast-to-coast voting.

"We would like to have everybody on board. We'd like to have a totally united party," he said.

Buoyed by a string of high-profile endorsements, including one from the Los Angeles Times, the Arizona senator sounded like a front-runner in talking to reporters, rejecting criticism of his conservative credentials from rival Mitt Romney and declaring Democrats Barack Obama and Hillary Clinton too inexperienced to be president.

Tuesday is the biggest day on the US electoral calendar for choosing Republican and Democratic candidates for the November presidential election, with contests in 24 different states in all parts of the country.

Asked if the Republican race would effectively be over on Tuesday, McCain said he had asked for divine intervention.

"From what we see in the polls, I think that there's a very good chance that it'll be over on Tuesday. But I think there's still a lot of undecided voters. But I'm hoping. ... The sooner we can get that done, the sooner I can go to work on uniting the party," he said.

Speaking later to the Republican Party Lincoln Day dinner in a Chicago suburb, McCain sounded more confident: "I think we may be able to wrap it up next Tuesday."