In an interview with The Irish Times, the Irish-American former Baltimore mayor and former Maryland governor, who is due to announce his candidacy on Saturday, said he had a tremendous amount of respect and admiration for the frontrunner.

“But an election, especially an election as important as the election of the next president of the United States, is not about what’s good for the Clintons or for that matter what’s good for the O’Malleys. It’s about what’s good for our country,” he said.

Mr O’Malley (52) is regarded as having positioned himself to the left of Ms Clinton (67) in order to appeal to the Democrats’ progressive base. “I don’t see myself as to the left or right of the other candidates in the party. I see myself as forward of them. I’m younger than they are. I therefore arrive at some conclusions on important issues in advance of the other candidates.”

Asked about the widely held belief, backed up by polling, that Ms Clinton’s candidacy is unstoppable within the Democratic Party, he said: “Every year there’s an unstoppable candidacy. Inevitable frontrunners are inevitable right up until they’re no longer inevitable.”

He said the Republican Party tended to “retread old names” while the Democrats tended to look to the future more. “In our party, there usually emerges a choice between the candidate that everybody knows and the new candidate of a new generation that none of us has ever really heard of much,” he said.