John McCain is considering making a pledge that he would serve only one term in the White House when he accepts the Republican nomination at his party’s convention next month.

A promise not to seek reelection after four years would be unprecedented in modern presidential politics and the idea is understood to be opposed by many senior figures in his campaign who fear that it would reduce his leverage with Congress and create chaos in the Republican Party.

Mr McCain, who has a habit of ignoring advice and bucking conventional wisdom, is attracted by its potential to transform a race that has already showed signs of tightening in recent weeks.

His Democratic opponent, Barack Obama, is presenting himself as a figure of generational change who can unite America by reaching out to independent and even Republican voters.

Mr McCain, who will celebrate his 72nd birthday while the Democratic convention is held this month, would be the oldest first-term US President ever. A pledge to limit himself to four years in office might go some way to assuaging concerns about his age.

Perhaps even more importantly it could help Mr McCain — who has his own strong appeal to independent voters — to distance himself farther from the partisan political divisions that have made Washington a byword for inaction and self-serving interests in recent years.

A one-term President would be able to devote all his time to tackling problems — including an ailing economy and two unfinished wars — rather than planning a reelection campaign. Supporters of Hillary Clinton, about a fifth of whom are already backing Mr McCain, could vote for him knowing that their candidate might run again in 2012.

His efforts to project a bipartisan message could be reinforced if he picks Joe Lieberman, 66, as his running-mate. The Connecticut senator was the Democratic vice-presidential nominee in 2000 but has emerged as a leading ally of Mr McCain, not least over their shared support for the Iraq war. Mr Lieberman would also be expected to make a pledge not to seek the nomination in 2012, leaving the field open for more conservative candidates.

Mr Lieberman’s liberal position on social issues such as abortion would alienate many Republicans but Mr McCain made plain this week that he would not hesitate to put a pro-choice candidate on the ticket.

Asked about picking Tom Ridge, a moderate Republican and former Governor of Pennsylvania, as his vice-presidential nominee, he told the Weekly Standard: “I think that the pro-life position is one of the important aspects or fundamentals of the Republican Party.

“I feel that — and I’m not trying to equivocate here — that Americans want us to work together. You know, Tom Ridge is one of the great leaders and he happens to be pro-choice . . . but that does not mean we exclude people from our party that are pro-choice.”

Mr McCain has blown hot and cold on the one-term pledge. It was originally included in his speech announcing his White House run in April last year but removed at the last minute after pleading from his advisers. In January he suggested that a four-year limit would make him a lame-duck President who would be “quacking on inauguration day”. At another public appearance in the same month, he also suggested: “If I said I was running for eight years, I’m not sure that would be a vote-getter.”

Last week Rick Davis, his campaign manager, declined to rule out Mr McCain delivering the promise at the convention. Appearing on Fox News Sunday, he said: “You’re going to have to come to the convention.”

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