By Paul Krugman – The New York Times.

It ain’t over until the portly gentleman screams, but it is, as intelligence analysts say, highly likely that Hillary Clinton will win this election. Poll-based models put her chances at around 90 percent earlier this week — and that was before the campaign turned totally X-rated.

But what will our first female president actually be able to accomplish? That depends on how big a victory she achieves.

I’m not talking about the size of her “mandate,” which means nothing: If the Obama years are any indication, Republicans will oppose anything she proposes no matter how badly they lose. The question, instead, is what happens to Congress.

Consider, first, the effects of a minimal victory: Mrs. Clinton becomes president, but Republicans hold on to both houses of Congress.

Such a victory wouldn’t be meaningless. It would avert the nightmare of a Trump presidency, and it would also block the radical tax-cutting, privatizing agenda that Paul Ryan, the speaker of the House, has made clear he will steamroll through if Mr. Trump somehow wins. But it would leave little room for positive action.

Things will be quite different if Democrats retake the Senate. Poll-based models give this outcome only around a 50-50 chance, but people betting on the election give it much better odds, two or three to one.

Now, even a Democratic Senate wouldn’t enable Mrs. Clinton to pass legislation in the face of an implacably obstructionist Republican majority in the House. It would, however, allow her to fill the Supreme Court seat left vacant by the death of Antonin Scalia.

Doing that would have huge consequences, for environmental policy in particular. In his final years in office, President Obama has made a major environmental push using his regulatory powers, for example by sharply tightening emission standardsfor heavy trucks.

But the most important piece of his push — the Clean Power Plan, which would greatly reduce greenhouse gas emissions from coal-fired power plants — is currently on hold, thanks to a stay imposed by the Supreme Court. Democratic capture of the Senate would remove this roadblock.

And bear in mind that climate change is by far the most important issue facing America and the world, even if the people selecting questions for the presidential debates for some reason refuse to bring it up. Quite simply, if Democrats take the Senate, we might take the minimum action needed to avoid catastrophe; if they don’t, we won’t.

What about the House? All, and I mean all, of the Obama administration’s legislative achievements took place during the two-year period when Democrats controlled both houses of Congress. Can that happen again?

Until the last few days, the chances of flipping the House seemed low, even if, as now seems all but certain, Democratic candidates in total receive more votes than Republicans. Partly that’s because G.O.P.-controlled state governments have engaged in pervasive gerrymandering; partly it’s because minority voters, who overwhelmingly favor Democrats, are clustered in a relatively small number of urban districts.

But a sufficiently big Clinton victory could change that, especially if suburban women desert a G.O.P. that has turned into the gropers-owned party. And that would let her pursue a much more expansive agenda.

There’s not much mystery about what that agenda would be. I don’t know why so many pundits claim that Mrs. Clinton lacks a vision for America, when she has actually provided an unusual level of detail on her website and in speeches.

Broadly speaking, she would significantly strengthen the social safety net, especially for the very poor and children, with an emphasis on family-related issues like parental leave. Such programs would cost money, although not as much as critics claim; she proposes, credibly, to raise that money with higher taxes on top incomes, so that the overall effect would be to reduce inequality.

Democratic control of the House would also open the door for large-scale infrastructure investment. If that seems feasible, I know that many progressive economists — myself included — will urge Mrs. Clinton to go significantly bigger than she is currently proposing.

If all of this sounds to you like a second round of what President Obama did in 2009-2010, that’s because it is. And why not? Despite Republican obstruction, Mr. Obama has presided over a remarkable rise in the number of Americans with health insurance, a significant decline in poverty and the creation of more than 11 million private-sector jobs.

In any case, the bottom line is that if you’re thinking of staying home on Election Day because the outcome is assured, don’t. Barring the political equivalent of a meteor strike, Hillary Clinton will be our next president, but the size of her victory will determine what kind of president she can be.