The president whom George W. Bush takes as a model is William McKinley. The man the president takes as a model is his father. But what if the president has chosen the wrong models? What if the models Mr. Bush has chosen don’t fit?

Richard Nixon, for example, wanted to be Woodrow Wilson. Lyndon Johnson wanted to be FDR. Ronald Reagan wanted to be Calvin Coolidge. Franklin Roosevelt wanted to be Theodore Roosevelt. Right now President Bush would be happy NOT to be Andrew Jackson, the only president to be censured. None of these molds fit the men who tried to squeeze into them. None is even close.

But the truth is that the president whom George W. Bush most resembles is not McKinley. It is Franklin Delano Roosevelt.

I can hear the gasps streaming through my computer terminal. From the left: How dare you compare the greatest president since Lincoln with the worst? From the right: How could you possibly confuse a man who is committed to individual responsibility with a man who recklessly wanted the government to do everything?

Tough. These two men may be the most alike of any two American presidents ever.

Both were born to aristocratic American families with English pretensions. Both were educated at New England boarding schools. Both went to Ivy League universities. Both failed to live up to the legends and records of accomplished family members in college. Both were marked by their failure to feel accepted by their peers in their undergraduate years. Both were playboys who slid through life on the basis of their personalities, and not their thoughts. Both married women who were more serious-minded than they were.

But there is more. Both were marked by personal crises – polio for one, potential alcoholism for the other – as adults. Both emerged from those crises as different men, in debt to women for the loyalty they displayed even when neither really deserved it. Both fell into politics because there was really nothing else they wanted to do. Both began in the shadows of their more famous relatives. Both eventually eclipsed their forebears. Both presided over major changes in the economy, the culture and the place America occupied in the world.

And then there is this: Both took ideologies they inherited to new places.

FDR, a Democrat, took the liberalism of TR, a Republican, and applied it more broadly and more aggressively, eventually claiming the term as his own and transforming not only the presidency but also the entire American government and the society outside Washington. Mr. Bush took the conservatism of his father and grandfather – a softer conservatism, it might be argued, than the Reagan brand, more rooted in restraint than in muscular Republicanism – and plotted a new course for conservatism, one aligned with religious institutions. Neither ideology was quite the same after FDR and W finished with it.

But the thing with both Franklin D. Roosevelt and George W. Bush is that they came to their ideologies the way the British acquired their empire – in a fit of absentmindedness.

These ideologies were, at best, inherited. Neither man gave extensive thought to the intellectual roots and traditions of the ideology he acquired, the way others acquire bad eyesight or a strong chin. Franklin Roosevelt was a liberal because Teddy Roosevelt was a liberal, and FDR above all admired TR. Mr. Bush was a conservative because Sen. Prescott Bush, the Connecticut Republican who was his grandfather, and George H.W. Bush, who is his father, were conservatives.

Then came the main chance. The trust-busting liberalism of TR offered few insights of value in the Great Depression, and so FDR, who had no particular compass, reached further. The same is true for Mr. Bush. The Taft Republicanism of his grandfather and the me-too Reaganism of his father offered few nostrums for the 21st century. Mr. Bush took his birthright and pushed it to new places.

Then why do these two men still seem so different, even to a columnist who is struggling mightily to argue that they are, in character and deed, more alike than different?

Perhaps it is because, in adopting and adapting his views, Roosevelt came to believe in the wicked-smart advisers who were leading him, including the Keynesians, and though the innards of the Roosevelt ideology had a messy beginning, they had a coherent end. It all held together by the end of the first term, and it provided a record to run on and a philosophy to govern by.

Not so in the Bush administration. And it is not only the president’s ideological opponents who are saying this. Indeed, there is more honest disagreement today among conservatives loosely under the Bush umbrella than there was at this point in his presidency among liberals under the FDR umbrella.

Once it reached full flower, and once the president signed on enthusiastically, New Deal liberalism was based on a theory of government and on a well-understood and well-articulated relationship between the state and individuals. Even its opponents – especially its opponents – agreed with this notion.

The same cannot be said for much of Bush conservatism. There remains honest debate among conservatives about the size of government, and honest disapproval of how Bush has expanded the role of government in the wake of Sept. 11 and Hurricane Katrina. There remains no clearly developed Bush philosophy that deals with the role of the state.

The difference is that liberalism under Roosevelt was a finished, polished ideology and philosophy of governance. Conservatism under Bush is still up for grabs, with various strains in struggle: There are Burkean conservatives, who feel the president doesn’t adhere sufficiently to the lessons and precedents of history; and economic conservatives, who are troubled about the big-state role Washington has assumed; and libertarians who are anxious about the role of social conservatives in the Bush chapel.

By this point in FDR’s second term it was clear what liberalism was in the Roosevelt era. At this point in Bush’s second term, there remains confusion, and contention, about what conservatism means in the Bush era. To succeed in history, and perhaps in the two years ahead, Mr. Bush must do what every impulse tells him not to do. He must become even more like FDR.