When Senator John McCain appeared at the Conservative party conference in Bournemouth last October as the presumptive next president of the US, the stars seemed fixed in the firmament for him. The myth of McCain appeared as invincible as ever.

His war story - a bomber pilot shot down over North Vietnam in 1967, held prisoner for five years and tortured - is the basis of his legend as morally courageous, authentic, unwavering in his convictions, an independent reformer willing to take on the reactionaries of his own party, an "American maverick" as he calls himself in his campaign autobiography.

The titles of his books reflect the image: Character Is Destiny, Why Courage Matters, and Faith of My Fathers. Defeat at the hands of George Bush in the battle for the Republican nomination in 2000, in which he was subjected to dirty tricks, completed his canonisation. The press corps so venerated him that he called them "my base".

McCain's political colleagues, however, know another side of the action hero - a volatile man with a hair-trigger temper, who shouted at Senator Ted Kennedy on the Senate floor to "shut up", and called fellow Republican senators "shithead ... fucking jerk ... asshole". A few months ago, McCain suddenly rushed up to a friend of mine, a prominent Washington lawyer, at a social event, and threatened to beat him up because he represented a client McCain happened to dislike. Then, just as suddenly, profusely and tearfully, he apologised.

Many Republicans who have had dealings with McCain distrust him (not just conservatives but traditional Republican moderates too). While taking rightwing positions on social issues such as abortion and gay marriage, his simmering resentment of Bush led him virtually to caucus with the Democrats in early 2001 (before September 11). Then, abruptly, he rushed to embrace Bush.

McCain's political advisers believe that he would easily be elected president in 2008, but fear that he might not capture the nomination. In 2000 he did not win a primary state where the voting was restricted to Republicans. So McCain decided to let the election take care of itself as he won over the party faithful. He campaigned enthusiastically for Bush in 2004. He sought to reconcile with the religious right, whose leaders he had called "agents of intolerance" in 2000.

McCain had belatedly taken the lead in opposing Bush's torture policy, an issue that could not be more personal for him. But after the supreme court last year declared Bush's secret tribunals for detainees and use of extreme interrogation techniques illegal, the president sought congressional approval of his version. At first, McCain fought Bush, but the right attacked him. McCain quickly capitulated, even agreeing to suspension of habeas corpus. Someone close to him explained to me that McCain calculated he could continue to play the issue when he became chairman of the Senate armed services committee in the next Congress. Asked about the chance that the Democrats might take control, McCain declared: "I think I'd just commit suicide."

As the neoconservatives abandoned Bush's sinking ship, McCain welcomed them aboard. "McCain began reading the Weekly Standard and conferring with its editors, particularly Bill Kristol," the New Republic magazine reported. And he hired a board member of the neocon Project for the New American Century, Randy Scheunemann, as his foreign-policy aide.

McCain positioned himself as consistently belligerent, even to Bush's right: in favour of bombing Iran and North Korea. He also proposed a "surge" of troops into Iraq, an idea gleaned from the neocons. If Bush had adopted the Iraq Study Group approach of diplomacy and redeployment, which McCain had assailed as "dispiriting", the right would have hailed McCain as a prophet with honour. However, importuned by the same neocons who had sold it to McCain, Bush seized upon the "surge".

McCain had trapped himself. He is now chained to Bush. As Bush's war has escalated, McCain's popularity has nose dived. Still the frontrunner for the Republican nomination, he might have made himself more acceptable to the base, but his political strategy has shattered his myth. Bearing the burden of Bush, he may have become unelectable.

· Sidney Blumenthal, a former senior adviser to President Clinton, is the author of How Bush Rules

sidney_blumenthal@yahoo.com