It is a cheap shot, but effective. Whoever came up with the riff “John McCain: Bush’s Third Term” has tapped into a potent message that is deeply damaging to the Arizona senator’s chances of reaching the White House.

The slogan has proven so popular that MoveOn.org ran out of its “Bush’s Third Term” bumper stickers long ago. Another poster has a picture of Mr. McCain’s face alongside the phrase, “The same old same old.” For Senator Obama, presenting himself as a young champion of change, what better way to dismiss a veteran rival than say he is just more of the same.

Mr. Bush’s personal standing has been so low for so long that any association with him is toxic. The latest Los Angeles Times/Bloomberg poll puts the president’s popularity at 23%, with nearly three-quarters disapproving of him. Little wonder that the latest hot Democratic election gizmo is an end of the George W. Bush years electronic key-ring that counts down the remaining days, hours, and seconds of the administration.

But to suggest that a McCain presidency would be little more than a continuation of Mr. Bush is a canard that causes many conservatives and many more observant liberals to laugh out loud. The Republican nominee is a loyal disciple to no one.

The true nature of the McCain/Bush relationship can be traced to the Republican primaries of 2000, where Mr. Bush was the establishment candidate and Mr. McCain the outsider. After Mr. McCain’s unexpected triumph in South Carolina, the Bush forces set him up for a beating.

While Mr. Bush stood silent, the Texas governor’s surrogates began inundating voters with lies and smears. It was suggested that the Arizona senator was gay, had fathered a black daughter (his adopted daughter Bridget is from Bangladesh), that Cindy McCain was addicted to drugs, and that his days in the Hanoi Hilton had left him mentally unstable.

Although there have since been many public acts of reconciliation between the president and Mr. McCain, the humiliation he endured at the hands of the president’s men continues to rankle the short tempered senator.

In the intervening eight years, Mr. McCain has shown both his loyalty to the president and his independence. At first shunning the Bush tax cuts because they favored the rich, Mr. McCain now believes that rescinding them while the country is on the threshold of a recession would be damaging.

On the environment, Mr. McCain has deep differences with Mr. Bush, starting with a belief that global warming is real and man made. While he disagrees with the president on drilling for oil in Alaska, Mr. McCain has come round to offshore drilling.

Mr. McCain’s most aberrant act, according to conservatives, was tampering with the Constitution in pursuit of campaign finance reform. Mr. McCain’s remark that “I would rather have a clean government than one where ‘First Amendment rights’ are being respected that has become corrupt,” was, for conservatives, a betrayal of first principles.

His second big break from conservatives, in advocating immigration reform that would allow millions of illegals eventually to win American citizenship, put him in agreement with President Bush but also on the side of many liberals. When the presidential debates come to discuss immigration, it will not be easy for Mr. Obama to paint Mr. McCain as a heartless xenophobe.

It is Mr. McCain’s devotion to the war in Iraq that causes Democrats to believe they have him on the run. What has been labeled “Bush’s War” would simply become “McCain’s War.” Did Mr. McCain not seriously declare that American troops may remain in Iraq for “hundreds of years”?

Yet there is no issue on which Mr. McCain and the president have disagreed as openly and as strongly as the Iraq War. Mr. McCain’s criticisms have not only been pointed and personal, but have led to changes in the way the war has been waged.

He was among the most vocal in blaming the president for allowing Donald Rumsfeld to mishandle the occupation. “As Lincoln and Truman demonstrated,” Mr. McCain told the Council on Foreign Relations, “American presidents cannot always leave decisions on matters of supreme national interest to their subordinates.” Mr. Rumsfeld’s hasty departure from the Pentagon had more to do with Mr. McCain’s moral condemnation than to the anti-war movement.

Offering a strategy in Mr. Rumsfeld’s absence, Mr. McCain advocated a significant increase in American forces in Iraq, an act of leadership that threatened to ditch his presidential campaign and tied his fate inexorably to the success of “the surge.” The president’s adoption of the surge was an admission by the president that his war policy was flawed and needed correcting.

Drawing on his experience as a prisoner of war, Mr. McCain has been as vocal as any Democrat on urging the president to outlaw torture by American forces and for the end of the prison camp at Guantanamo Bay. Again, Mr. McCain has set himself up as a moral arbiter, unafraid to find himself in a minority of Republicans on such a fundamental issue.

Mr. McCain has been trying gently to distance himself from the president in order to win independent voters, but he risks making conservatives even less happy. The key to his salvation lies, perhaps, in the fine print of a recent CBS News poll that showed that at least half of Americans do not buy the Bush parallel and know full well that in the White House Mr. McCain will be his own man.

nwapshott@nysun.com