With Donald Trump in the White House and Congress bitterly divided, setting economic policy has been a haphazard, piecemeal business. In December, congressional Republicans rammed through an overhaul of the tax code that was so rushed, it’s still not clear how parts of it will work in practice. This week, the leaders of the two parties in the Senate agreed on a two-year budget deal that will boost spending considerably. Despite objections from lawmakers on both sides, the measure passed both the House and Senate early on Friday morning, and was signed by Trump soon after.

As part of the deal, the Pentagon, which has spent years complaining about spending restraints that were introduced in 2011, will get an eighty-billion-dollar increase in its budget for this year, and an extra eighty-five billion dollars for next year. In return for approving this boost to defense spending, the Democrats get a slightly smaller increase in other forms of discretionary spending, which have also been squeezed since the Obama years. (This increase covers everything from the National Park Service to Pell Grants to Section 8 rental subsidies for poor people.) All told, the deal authorizes about three hundred billion dollars in new spending over the next two years.

This figure needs to be placed in context. Gross domestic product, the broadest measure of all the goods and services that the U.S. economy produces in a year, is currently about twenty trillion dollars. As a share of G.D.P., which is the best way to look at federal spending, the increases in this budget deal amount to about three-quarters of one per cent each year. But that comes on top of some hefty front-loaded tax cuts. According to an analysis by the Joint Committee on Taxation, the G.O.P. tax bill will reduce tax payments by about a hundred and thirty-five billion dollars in the 2018 fiscal year, and by two hundred and eighty billion dollars in the 2019 fiscal year. To be sure, most of those tax cuts were for the benefit of businesses and rich people, but the result is still extra money in the economy that can be spent. If you add it to the additional spending in the budget deal, you’ve got a pretty big “Trump stimulus.”

How big? It’s not easy to give an exact answer. Jason Furman, who was the chairman of the White House Council of Economic Advisers during the second term of the Obama Administration, pointed out to me on Wednesday that the budget deal involves “budget authority” rather than actual outlays, which as a result means that the spending increases it authorizes may take longer to be disbursed, especially at the Department of Defense. The figures also need to be adjusted to take account of certain other policy measures, such as tax extenders and sequester relief, which were in effect during 2017. Furman said he hasn’t yet done all the math, but he provided a “very rough first guess” that the over-all effect of the tax bill and the spending deal would be about 1.25 per cent of G.D.P. for this calendar year, and two per cent for the next.

That would be a substantial stimulus. It would be larger, for example, than the Economic Stimulus Act of 2008, which George W. Bush’s Administration introduced to try to head off a slump following a big fall in the real-estate market. That stimulus, which came in the form of a tax rebate, amounted to about one per cent of G.D.P. (It wasn’t enough to head off a recession. In fact, the National Bureau of Economic Research subsequently said that the recession had already begun in December, 2007.)

The Trump stimulus isn’t as big as the Obama stimulus of 2009 through 2011, which most Republican senators and congressmen vigorously opposed. That package, which consisted of a mix of spending and tax cuts, totalled about two per cent of G.D.P. each year. But, in February, 2009, when it was enacted, the economy was suffering through the deepest recession since the nineteen-thirties. The unemployment rate was 7.8 per cent, and G.D.P. was plummeting. If ever there was a textbook case of an economy crying out for a stimulus, that was it.

Today, by contrast, the economy is in the ninth year of an economic recovery that began in 2009. G.D.P. is growing at an annual rate of close to three per cent, and the unemployment rate stands at 4.1 per cent. Many economics textbooks say this is the sort of environment in which the government should be balancing its books, and perhaps even paying down debt, like a family salting away money for a rainy day. That’s what the Clinton Administration did during the late nineteen-nineties, when the national debt was much smaller than it is today.

The Republicans and Trump are embarked on the opposite course—confirming that the G.O.P.’s devotion to deficit reduction, which in 2011 prompted members of the Party to refuse to raise the debt ceiling, is purely cynical. Of course, we already knew this. The Reagan Administration and the George W. Bush Administration both raided the public purse to finance big tax cuts, and left the deficit much higher than they found it. The Trump Administration is merely following suit.

Even before this week’s spending deal was reached, the Committee for a Responsible Federal Budget, a hawkish fiscal watchdog in Washington, had predicted that, as a result of the tax bill, the budget deficit would rise to a trillion dollars in 2019, which would be about 4.8 per cent of G.D.P. (In the 2017 fiscal year, which ended on September 30th, the deficit was $665.7 billion, or about 3.5 per cent of G.D.P.) “Based on what we know,” the group said on Wednesday, “the budget deal would increase next year’s deficits to roughly $1.2 trillion. Annual deficits would remain over $1 trillion indefinitely.”

Furman agrees with the C.R.F.B. that we should be worried about this. He pointed out that higher deficits put upward pressure on interest rates, which can choke off capital investment and lead to falls in stock prices. In addition, he noted, it is advisable to bring down the deficit during good times, so that there is room to raise it in bad times.

Other economists, including some liberal ones, are less concerned about the deficit implications of the spending deal. “There’s a kind of recklessness to Team Trump that could kind of rebound to the benefit of people,” Jared Bernstein, who was Joe Biden’s top economic adviser during the Obama Administration, told Jim Tankersley, of the Times, earlier this week. “As long as there is still slack in corners of the labor market, then this kind of fiscal stimulus of the economy near full employment is a kind of test I support.”

But will the financial markets even allow the test to proceed? On Wednesday, investors reacted to the budget deal by selling Treasury bonds, interest rates rose, and the Dow Jones Industrial Average fell by more than a thousand points. If rates rise much higher, and the stock market’s fall turns into a rout, the Trump-G.O.P. stimulus could be subverted before it even gets going.