The election offered an imperfect choice: A realistic Democrat who lacked vision, or a visionary Republican who lacked realism. Now that the nation has chosen the visionary option, the challenge for the rest of the political establishment -- for congressional leaders, responsible administration officials and news organizations -- will be to avoid getting swept up in the excitement of the victor's bold ideas. We know this leader has the Vision Thing. But what about reality?

President Bush's news conference last week illustrated the nature of this challenge. He came bouncing into the briefing room, teasing his questioners and laughing at his own grammatical errors, and he gave Washington what it most loves: A Really Big Agenda. He would stick with his audacious foreign policy. He would overhaul Social Security. He would rethink the tort system and the tax system.

A Really Big Agenda is manna for almost everyone within a mile of the White House. In the wake of Bush's news conference, White House commentators were penning their Important Articles; senators were plotting their Important Bills; and think-tankers were basking in the delicious sense that their policy briefs were going to matter. Tax reform? The lobbyists on K Street could hear their cash registers dinging. Social Security privatization? At last AARP could justify its vast office in Washington.

But the sane reaction to Bush's big agenda ought to be: Whoa! Not so fast. You've already shown you have the guts to do big things -- radical tax cuts, a radical war in Iraq. What we need you to demonstrate now is that you can blend vision with realism.

By that yardstick, Bush's ebullient news conference was a failure. On foreign policy, he had a chance to win over allies by apologizing for mistakes in Iraq and by having the wisdom to admit that the universal urge to be free does not actually guarantee that freedom will triumph. Instead, Bush bubbled on about his election mandate and his political capital.

Before the election, Bush had a political excuse for his refusal to apologize: The Kerry campaign would have replayed his words in thousands of TV spots. But now the political excuse is gone, and Bush's triumphalism is self-defeating. A realistic president would see that you can't face down Iran over its nuclear program without European support, and you probably can't stay the course in Iraq without international backing either. A realistic president would climb off his high horse so that Europeans could descend from theirs.

The same is true of domestic policy. The election now over, Bush had a chance to speak honestly about the budget: to acknowledge that his vision of a low-tax economy has been pushed unrealistically. His grand tax-reform agenda offered the perfect opportunity for elegant retreat: He could have promised to raise revenue by closing tax loopholes. Instead, the president insisted that loophole closings would be offset by yet more tax cuts elsewhere. And he repeated the Big Lie that he has a five-year plan to halve the budget deficit.

My colleague Jonathan Weisman called him on that one. The deficit in fiscal 2004 was $413 billion; under the plan that the administration submitted to Congress, the deficit in 2009 will be $258 billion. Moreover, this non-halving is based on shaky assumptions: It excludes emergency appropriations for commitments in Afghanistan and Iraq, and it pretends that Congress will restrain its spending habit. But the biggest problem with the Big Lie is that the five-year time frame is irrelevant. The real fiscal challenge lies beyond, with medical inflation that we can't seem to control and the retirement of the baby boomers.

Bush ventured in his news conference that economic growth could fix the long-term budget problem. Again, he fails the test of realism. According to calculations by Alan Auerbach of the University of California at Berkeley and Peter Orszag and William Gale of the Brookings Institution, current trends point toward a budget deficit in 2040 that will be an astonishing 20 percent of GDP -- five times as much as the current one. The main driver is not Social Security, which Bush wants to reform. It is the health programs for the poor and old, whose cost increase is projected to be three times as big as the increase for Social Security.

A realistic president would confess that pro-growth policies cannot plug that future deficit. According to Robert Litan of Brookings, tort reform might boost growth by 0.1 percent per year, an increase that scratches the surface of the fiscal problem. The same goes for the government's other options, such as trade liberalization or regulatory reform. A radical effort to discipline health spending would have the biggest impact. But even that would cut the 2040 deficit by just a few percentage points of GDP. Barring some kind of miracle, it's impossible to imagine a solution to the future budget crunch that does not involve higher taxes.

It's nice to have a president with bold ideas, and many of Bush's ideas are attractive. But boldness can be dangerous if it's not rooted in reality. We don't need this president to prove yet again that, unlike his father, he has the Vision Thing. We need him to show that, like his father, he has maturity and judgment.

mallabys@washpost.com