The spring slowdown in the economy has further exposed President Obama’s greatest political vulnerability going into his reelection campaign and forced him into a more urgent and aggressive debate with Mitt Romney on the central issue of the presidential election.

On Wednesday, Romney pounced. In a speech before business leaders, he not only blamed Obama for not turning around the economy but also indicted the central philosophy underlying the president’s efforts.

Obama is expected to return fire on Thursday in what the White House is calling a major speech that will characterize Romney’s economic philosophy as a return to the flawed policies of the George W. Bush years.

The intense exchange represents more than just campaign rhetoric; it lays out in stark terms the divergent approaches the two men propose to take toward mending the economy. And while the proposals are familiar, the implications of the debate suddenly seem more urgent.

“I think this election is a watershed election, which will determine the relationship between citizen and enterprise and government,” Romney said Wednesday.

Some of the political frenzy is driven by economic forecasts that continue to lower expectations for economic growth amid a lull in hiring, weak domestic economic data, and new threats from Europe and emerging markets.

Romney has argued against emergency steps by the government to bolster short-term growth, such as increasing federal spending. He has instead called for policies that he believes would improve the economy structurally over the long term, even if they did little to accelerate recovery in coming months.

In broad terms, those policies include keeping tax rates low for all Americans, eliminating Obama’s health-care overhaul and scaling back other regulations, and reducing federal spending.

Romney’s theory is that keeping tax rates low would spur investment in new businesses, thereby increasing economic growth and perhaps tax revenue itself. He believes that rolling back regulations would reduce the cost of doing business and make the United States more competitive.

“The president’s team indicated that if we passed their stimulus of $787 billion, borrowed, that they’d hold unemployment below 8 percent. We’ve gone 40 straight months with unemployment above 8 percent,” Romney said Wednesday at the Business Roundtable, a trade group in Washington.

He added: “If you look at his record over the last 31 / 2 years, you will conclude, as I have, that it is the most anti-investment, anti-business, anti-jobs series of policies in modern American history.”

Obama has a different policy prescription.

In the short term, he wants to do more of what he did at the beginning of his tenure: use taxpayer money to hire people to build roads and bridges and give money to states and localities to hire more teachers, among other measures.

“Mitt Romney’s plan for the economy is great — if you’re a millionaire like Mitt Romney. He wants to turn back the clock and return to trickle-down economics, which lost private-sector jobs, hurt working families, and got us into this mess in the first place,” Rep. Chris Van Hollen (D-Md.), an Obama surrogate, said Wednesday. “Moving backward will do nothing to put Americans back to work or boost our recovery.”

On Thursday in Cleveland, Obama is expected to promote policies that he believes will sustain economic growth over the long term, such as government investment in energy and education.

And he’s likely to push back against the GOP assessment that he has failed.

“Because folks are still hurting right now, the other side feels that it’s enough for them to just sit back and say, ‘Things aren’t as good as they should be, and it’s Obama’s fault,’ ” the president said Tuesday in Baltimore. “You can pretty much put their campaign on a tweet and have some characters to spare.”

One issue on which the differing philosophies most strongly emerge is taxation, even though both candidates agree that taxes are a key component of any recovery. The two men agree that lower tax rates would be desirable, with the lost revenue made up in part by eliminating numerous tax deductions.

But Obama believes the government must take in more tax revenue, in particular from the wealthy, to pay for the spending he views as critical to long-term economic growth. That way, he argues that the earnings generated by companies should be flowing to a broad swath of workers rather than being concentrated just among top executives.

In his remarks Wednesday, Romney painted a dire portrait of the economy during a second Obama term. He warned that Obama would “stifle” energy resources in coal, oil and natural gas, impose regulations that would raise the cost of doing business and raise taxes on the wealthiest Americans.

Romney argued that raising taxes to spend more is counterproductive — that higher levels of taxation discourage investment and make the United States less competitive. He says it is better to cut government spending so not as much tax revenue is necessary.

“He lived the economy. He knows it inside out,” said Rep. Paul Ryan (R-Wis.), chairman of the House Budget Committee and a prominent Romney supporter. “Our views are based on real-live evidence. . . . So he is living proof of that evidence.”

The problem for Romney is that some economists doubt that it is possible to cut spending enough to match lower levels of tax revenue.

Robert Bixby, head of the Concord Coalition, a nonpartisan group arguing for reducing deficits, said Romney could have trouble capping federal spending, providing the benefits Americans want and avoiding unsustainable budget deficits.

“If you can make that work, fine,” he said. “Where you run into a problem is when the tax-cut promises are quite specific and the spending-cut promises are kind of vague.”

The irony is that at the end of the year, unless Congress acts, both Obama and Romney will get part of what they say want — and what they don’t want. Taxes will rise for almost all Americans as the George W. Bush tax cuts expire, while deep spending cuts will take effect under an agreement Congress reached last year to reduce deficits.

Mark Zandi, an economist with Moody’s Analytics, said it will be most important for policymakers to delay some of those tax increases and spending cuts to avoid hurting an economy that is still in recovery.

“I don’t think we want to have a fiscal head wind in our face going into 2013,” he said.

Staff writer David A. Fahrenthold contributed to this report.