WASHINGTON -- Barack Obama was elected, in part, as the antidote to ambition. Unlike John F. Kennedy, who campaigned against the golf-playing complacency of the Eisenhower era, Obama appealed to a nation weary of large national exertions -- a nation longing for a normalcy beyond the wars, hurricanes, floods and assorted plagues of the Bush years.

Yet headed toward the inauguration, the scale of Obama's ambition is becoming evident.

I am not referring to his stimulus plan, which tends toward the limited and conventional. Obama's tax cuts are designed to improve household cash flow. But it is the iron rule of stimulus packages that temporary tax cuts have temporary results -- like coffee instead of a meal for a hungry man.

Obama's proposed infrastructure spending has the Lincolnian appeal of fostering "internal improvements" -- raising the prospect that public spending might have some lasting public benefit. But there is a fine line between infrastructure and pork -- the line between building a bridge, which might promote economic growth, and painting a bridge, which merely provides a few jobs for a few weeks. And Congress is not particularly known for its navigation of fine lines.

The Obama economic plan is expensive but hardly revolutionary -- much less "socialist." In reality, American presidents have few levers long enough to shift a continental economy. During an economic winter, they shovel the snow -- and then take credit for the spring.

But there are two other areas where Obama's campaign pledges are proving ambitious.

First, Obama proposes to require all but the smallest businesses to provide health coverage to employees -- or pay a tax. He would also create a government-run insurance plan similar to Medicare that would compete with private companies to cover the uninsured. The problem is this: Because government can impose price controls, it can make the public option cheaper. Companies, tired of dealing with complicated health care burdens, would have an incentive to drop employees from coverage, and uncovered individuals would have an incentive to join the public system -- achieving universal nationalization of health care by small steps.

In reacting to this approach, Republicans face a difficult, defining moment. This is not 1994, when opponents, offering no alternatives, killed the Clinton health reform. Many businesses are sincerely discontented with the current employer-based system.

But predominantly publicly run health care is an ideological red line for Republicans. In other instances where the middle class has become dependent on government for its health care -- witness Britain -- the conservative case for individual responsibility and limited government has been fundamentally undermined. People hold tightly to the security of their benefits even when treated by a health system with surly incompetence. Not even the most compassionate conservative is going to accept government control of 16 percent of the economy.

If Obama's proposal demonstrates genuine neutrality between public and private health options -- empowering individuals to make a free choice -- it could gain significant Republican support. If the plan is an intermediary step toward a single-payer system, Obama can expect a serious fight, even from a weakened opponent, because the deepest values of American conservatism will be at stake.

Second, the scale of Obama's environmental ambitions has been highlighted by the current economic crisis.

It is another iron rule that prosperous, confident nations do more for the environment than economically struggling ones. And this sets up a conflict between Obama's urgent environmental diagnosis -- a cumulative scientific case for serious, possibly catastrophic climate disruption -- and the economic and political realities of the moment.

The centerpiece of Obama's environmental approach is an "economy-wide cap-and-trade program," designed to dramatically limit greenhouse gas production. But this would act as a large tax on the use of fossil fuels -- in an economy where falling energy prices have been one of the few sources of good news.

If Obama plows ahead with an aggressive cap-and-trade system, Republican and Democratic opponents -- focused exclusively on jobs -- will find plenty of excuses for legislative inertia. If he phases in a system too slowly, it will undermine his own arguments for urgency. If he abandons a cap-and-trade system in favor of investing in eco-infrastructure -- a more efficient energy grid, weatherizing public buildings -- he will get what he wants, and also get slammed for betraying a serious commitment.

During the campaign, I sometimes criticized Obama for lacking specificity and ambition. But as the specifics emerge, the ambitions of his campaign pledges are ever more clear.