Well, it's already happened. Barely two weeks into the job and President Barack Obama has compromised fundamental principles, timorously caved in to Republicans and conservative Democrats in the Senate and lost control of his agenda.

Or ... wait. Maybe it's the case that, a mere two weeks into the job, President Obama has already changed the country's direction in remarkable ways. He's on the verge of a massive political victory when the Senate passes the stimulus package tomorrow, as expected, and the Republicans are apoplectic and divided and intellectually bankrupt.

Which is it? Friends, I usher you on a tour of the liberal mind.

OK, what I'm about to say isn't true of everyone, of course. But there is a general thing: liberals are happy being unhappy. Or worrying. We're (I very much include myself) big worriers. With reason: history teaches that the tide of change doesn't always flow in our direction, especially in recent years. I know a lot of people who couldn't quite believe that America could elect a man like Barack Obama, and still didn't quite believe it after it happened.

In addition, there is a general tendency to accentuate the negative. Partisans of both sides focus on what has been lost in compromise, but there is a crucial difference in the quality of complaint. Conservatives tend to look upon compromise and shout: "Betrayal!" Liberals have more often tended to sigh: "Well, I figured as much." The blogosphere has given liberalism an often necessary jolt of the former disposition, but it's still the general reflex of the liberal mind (again, including my own) to assume the worst and nod knowingly as it inevitably happens.

Well, today, I announce my emancipation from such habits. Goodbye to all that. The stimulus bill, imperfect as it is, does indeed represent an enormous political victory for Obama. For reasons tactical as well as substantive, liberals ought to declare victory and dance on the vast empty tundra that is the Republican present.

Think back. Two months ago, people were talking nervously about a stimulus package worth about $400bn. Now? Assuming the Senate and House of Representatives more or less split the difference between their two versions of the bill - they will likely iron those out this week and vote on the final passage of the new product by the week's end - we're talking twice that, with at least $500bn in new spending (the rest is tax cuts). That is, by some distance, the largest public spending bill ever conceived in the US.

Republicans are in disarray. First, this approach goes against everything they believe. Second, they are suddenly losing an argument that they thought they were winning. To hear cable television tell the story last week, they had Obama on the ropes. Support for the package was allegedly sinking like a stone in the country. Then he goes out and gives a grand total of one speech, not even one of his better ones, and bam, suddenly they're losing. They must be absolutely irate - and privately very, very nervous about the future.

And yet, I hear a lot of liberal commentary about what a stinker the Senate package is. Well, as people should know, that's the Senate. That is how it's built, and that is how it works. In early 2001, George Bush proposed a $1.6 trillion tax cut. That April, the Senate cut $450bn out of it. Moderate liberal senators then put the brakes on legislation they saw as too conservative, just as moderate conservative senators last week did the same to legislation they thought too liberal. We can like it or not like it, but it's what the Senate was designed to do in the first place. Indeed, from a purely constitutional perspective, the Senate played its role here appropriately. This should not have surprised anyone.

Now, to be fair, the big concern of liberals who are unhappy with this bill - they wanted it to be larger, and less focused on tax cuts - is the central question of whether it will work. They say, this is our best shot in 30 years at showing that government can be part of the solution, and it damn well better show that. They're doubtful that this bill can.

Time may prove them right, but two points: a) then again, it might not, because who can really say, and b) in any case, this bill is not the Obama administration's only chance to do something about the economy. Treasury secretary Tim Geithner is rolling out a plan today to get credit flowing and protect homeowners. Soon, the administration will present a proper budget, in which it can signal priorities about things like transport and the greening of the economy, which are multi-year projects in the best of circumstances.

Liberals should press the administration for the most progressive outcome possible. That's fine and laudable. But at the same time, let's understand that they got about 80% of what they wanted here, and getting 80% of what you want is awfully rare, in politics or marriage or at the office or anywhere.

I'm nervous, too, about whether the bill will work. But meanwhile, its impending passage sets the country on a dramatically different course to the one it's pursued over the last eight years (the last 30, really). To me, that's hardly a stinker. In fact, it smells rather nice. I love the smell of stimulus spending in the morning. It smells like ... victory.

• Michael Tomasky is editor of Guardian America. Read his blog at Guardian.co.uk/commentisfree/michaeltomasky