The seeds of this ballooning debt load are already taking root — the Treasury Department said last month that it expects to borrow $955 billion this fiscal year and more than $1 trillion in both 2019 and 2020. That money appears set to get only more expensive to borrow, as the Federal Reserve looks to continue raising interest rates and investors demand higher returns from an increasingly debt-laden government.
Indeed, the borrowing spree is contributing to recent volatility in financial markets, as investors fret that the additional fiscal stimulus, paired with a strengthening economy, could fuel inflation and translate into higher interest rates more quickly than anticipated. Major stock indexes dropped sharply again late Thursday afternoon, falling into a market correction, or a drop of more than 10 percent from their peak, largely on comments from the Bank of England that it might raise interest rates sooner and higher as it looks to fend off possible inflation. Investors poured into bonds in a flight to safety, pushing the yield on the 10-year Treasury bill to a four-year high of 2.88 percent.
For many Republicans, backing the budget agreement is a break with conservative fiscal orthodoxy that carries risk going into the midterm elections. The party’s professed commitment to limited government and deficit reduction helped Republicans regain control of the House and Senate during the Obama administration and also helped President Trump win election, with the candidate promising to get federal spending under control.
Last May, the Trump administration released a budget projecting the United States would swing from a deficit of $440 billion in 2018 to a surplus of $16 billion in 2027. The budget called for deep cuts to domestic programs and a robust increase in military spending.
Instead, the opposite has happened as Washington has opened the spending spigot in ways once unimaginable to the Republicans’ fiscal discipline wing.
Some budget experts have pointed out that the budget deal, which Mr. Trump supports, is more costly than the fiscal plan that Hillary Clinton proposed during her 2016 presidential campaign.
Paul Winfree, an economist at the conservative Heritage Foundation, calculated that the 2019 base level of nondefense discretionary spending surpasses what President Barack Obama sought in his final budget request.
“This is getting comical,” Mr. Winfree, who recently left a job as an economic adviser to Mr. Trump, said in a post on Twitter.
While the current pact is only for 2018 and 2019, those who preach fiscal restraint fret that the spending spigot is unlikely to be shut off after that. The Committee for a Responsible Federal Budget projects that the two-year deal would set the stage for $1.7 trillion in additional deficits over the next 10 years because Congress will be operating from a higher budget baseline and will be unlikely to adopt a fresh round of austerity.
“This is the new baseline,” said Steve Bell, a senior adviser at the Bipartisan Policy Center and former Republican staff director of the Senate Budget Committee. “With this and the tax cuts, a balanced budget becomes a pipe dream.”
Republican lawmakers are re-entering a “deficits don’t matter” phase, like the ones that persisted during the Reagan and Bush presidencies. That has conservative economists queasy, as they look at a government budget that is only going to face more stress as an aging population turns to Medicare and Social Security and drives up spending levels.
“Republicans were very concerned about debt and deficits during President Obama’s two terms in office. Then their concern kind of evaporated,” said Michael R. Strain, an economist at the conservative American Enterprise Institute. “I do think as an economic reality, deficits do matter.”
Mr. Strain said the implications of the nation’s debt trajectory included significantly higher interest payments, a loss of confidence in American Treasury bonds and less room to maneuver in a recession.
The White House’s Office of Management and Budget is expected to release its fiscal 2019 budget next week. Raj Shah, a White House spokesman, said the budget outline would “outline a path toward fiscal responsibility” and that Mr. Trump remained concerned about fiscal discipline.
“He is concerned about spending in Washington,” Mr. Shah said. “He’s expressed that for years.”
Last week the Treasury Department said the United States will need to borrow $441 billion in privately held debt this quarter, the largest sum since 2010, when the economy was emerging from the worst downturn since the Great Depression.
Left-leaning economists are also scratching their heads at the speed at which Republicans seem to be disavowing their commitment fiscal restraint.
“I think it’s a little bit surprising and puzzling,” said Jason Furman, a former chairman of Mr. Obama’s Council of Economic Advisers, noting that trillion dollar budget deficits is uncharted territory during a period of full employment. “We’ve never had anything like that outside of a war or a recession.”
Mr. Furman said it would be wise for lawmakers to find some offsets for their spending spree.
For their part, Republican leaders who back the budget agreement are focusing on their desire to bolster military spending. They insist they would like to cut spending on programs such as Medicare and Social Security, the biggest drivers of the debt, if the political will were there.
“We’re going to work on other entitlement issues this year,” Speaker Paul D. Ryan said on the Hugh Hewitt radio show on Thursday. “It’s the entitlements that need to be reformed if we’re going to deal with the debt.”
But Senator Mitch McConnell of Kentucky, the majority leader, has ruled out addressing entitlement programs this year.
To Republicans who oppose the spending bill, the days of Tea Party activists railing against the growing debt seem like ancient history.
“We’re living in an unprecedented time from the standpoint of spending, and it’s very concerning,” said Senator Bob Corker, a Tennessee Republican who considers himself a deficit hawk.
Despite the disappointment of Republicans such as Mr. Corker, the desire to reduce deficits could return.
A fight between Republicans and Mr. Obama over federal spending and deficits ultimately led to passage of the 2011 Budget Control Act, whose spending caps would be lifted as part of this budget deal.
Mr. Strain, of the American Enterprise Institute, suggested that for deficit reduction to become a prevailing policy for Republicans, another Democratic president might be required.
“We’ve seen that movie before, and I don’t see any reason to think we won’t see it again,” Mr. Strain said.