Never mind the old centrist talk of George W. Bush as a compassionate conservative or the new centrist talk of John Kerry (search) as a nuanced lawmaker who always sees both sides.

In action, these men represent the core of their parties, or at least enough of it to offer voters crisp choices in the fall.

With the Democratic primary race decided, Kerry can be expected to edge away from the rhetoric his party's activists on the left most want to hear, and engage President Bush in a great battle for the middle. Already, he's begun branding some of his plans as a middle-class tax cut, and on Wednesday he rolled out a "tax fairness" package.

But when the dust settles, it will remain a good bet that income taxes will be lower overall in a Bush administration than in a Kerry one, that Kerry will do more to push for health insurance than the Republican, and that the social issues of the day will be tackled along familiar ideological lines no matter who wins.

Take abortion. Kerry says flatly he'll only nominate Supreme Court justices who agree with his views in favor of abortion rights. Bush, who's had no Supreme Court vacancies to fill, opposes most abortion rights and has a record of putting forward conservative judges for lower benches.

The choices are distinct right down the alphabet, to Social Security if not beyond. Partially privatize the retirement program? Yes, says Bush. No, says Kerry. No maybes there.

And sentiments that seem close on the surface may in fact be a gulf.

Both are against gay marriage (search), but Bush wants to amend the Constitution to ban it while Kerry would go most of the distance in granting gay couples the civil rights of marriage, if not the certificate.

For all that, there is no perfect predictor of the actual choices Americans have, because much is driven by circumstance.

There is no knowing where America would be, and what the president would be doing, absent the terrorist attacks that transformed priorities, brought on war and spawned the expanded federal bureaucracy that is the Homeland Security Department (search).

Recession ruins economic assumptions that candidates use in saying how they will pay for what they want to do.

And the Republican objection to gay marriage, couched in language defining marriage as a union of man and woman, sat quietly in the party platform, alongside other shelved items put there for the benefit of the GOP's activist right, until courts and civic officials defied the status quo and prompted Bush to call for the constitutional ban.

As for Kerry, he brings a 19-year voting record to the table as a Massachusetts senator. But his forte has been in investigating malfeasance, not in crafting legislation, and only so much can be read into votes when calculating what he would do as president.

"The problem with him is that he's never been an executive, so you don't know ... how he actually comes down and makes a decision," said Elaine Kamarck, a Harvard professor who advised Democrat Al Gore's 2000 presidential campaign. "By the time you get to a vote, things are pretty washed over."

Even so, Kerry has not often strayed from mainstream Democratic thinking, or what Kamarck calls the "Clintonian consensus" - the shaded liberalism of Bill Clinton that made room for certain principles associated with the right, such as tough stands on defense, crime and the budget.

"My sense is Kerry would move the country in a very different direction than Bush has," said Brown University political scientist Darrell West. "There's a pretty clear choice for Americans in terms of the role that government would play."

Both commit billions to health care but in different ways. Bush's answer to the growing number of uninsured comes in the name of helping individuals make a choice - letting them set up tax-favored medical savings accounts, for example, and giving them subsidized alternatives to Medicare under the new prescription drug program for the elderly.

Kerry faults the drug benefit, saying it will deprive the elderly of choice by steering them away from Medicare. He stops well short of a government-run universal insurance system but proposes to expand affordable coverage with an ambitious series of subsidies and tax incentives geared to children, the unemployed, small businesses and others.

Similarly on the environment, Bush looks largely to the market to create opportunities to reduce greenhouse gases, run cleaner cars and develop the technology to reduce other forms of pollution without constraining the economy.

Kerry hews closer to regulation of industry and to the negotiation of international treaties, while also proposing to spend heavily on clean technology. He holds out the possibility - but not, notably, the promise - of joining the Kyoto global warming treaty (search) rejected by Bush.

On education, Kerry supported the overhaul of school standards promoted by Bush and backed by both parties in Congress. But now he questions a centerpiece of the reforms, saying students, parents and schools are chafing under a regimen that places too much emphasis on test scores for measuring student achievement. And he says Bush hasn't put nearly enough money into the new system.

He would raise taxes on the rich by revoking the portion of the Bush tax cuts that went to those making over $200,000. On Wednesday he pitched his plans for health insurance and tuition tax credits as a deeper tax cut for middle-income Americans than anything Bush gave them.