John McCain is now looking beyond his immediate task of finishing off Republican rivals towards the more daunting challenge of dimming the stellar power of Barack Obama in this autumn’s general election.

Although he continues to be chased by Mike Huckabee, Mr McCain’s hat-trick of wins in the Potomac region on Tuesday have confirmed his position as the front-runner for his party’s nomination.

After months in which Republicans have trained most of their fire at Hillary Clinton, Mr McCain appears to have decided that she is now no longer his most likely opponent. Although not mentioning Mr Obama by name, he picked on a word that falls from the Democrat’s lips so often that it has become almost a trademark.

“Hope, my friends, is a powerful thing,” Mr McCain said, “[But] to encourage a country with only rhetoric rather than sound and proven ideas that trust in the strength and courage of free people is not a promise of hope. It is a platitude.” He added: “I do not seek the presidency on the presumption that I am blessed with such personal greatness that history has anointed me to save my country in its hour of need.”

Yesterday he acknowledged that his remarks had been aimed at Mr Obama: “There’s going to come a time when we’re going to have to get into specifics,” he said, adding that this was a quality “singularly lacking” from Mr Obama’s speeches.

The two senators have clashed before – in early 2006 – when Mr Obama withdrew from a promised cross-party effort to secure ethics reform. This prompted a letter of withering sarcasm from Mr McCain, thanking him for “disabusing me” of the idea that his assurances had been more than “typical rhetorical gloss routinely used in politics to make self-interested partisan posturing appear more noble”.

Although they later joked about their “pen-pal relationship”, their dispute two years ago may yet prove to be a significant pointer in a contest between candidates who appeal to independent and crossover voters.

That is where similarities end. If Mr Obama wins his party’s nomination, the November election will be presented as a choice between callow youth and grizzled experience – there is a 25-year age gap between them – Mr Obama’s “audacity of hope” against Mr McCain’s world-worn humility.

David Plouffe, Mr Obama’s campaign manger, said yesterday that while his candidate did not believe in “blind optimism”, there was “a lot of hope” to be found, including in the swing state of Virginia, where he won more votes on Tuesday than all the Republican candidates combined.

Mr McCain, struggling to overcome conservative scepticism, acknowledged yesterday: “We do have a lot of work to do – we all agree with that.”

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