Republicans are working to tag Democratic presidential front-runner Hillary Clinton as a big spender, adding up her proposals and finding more than $1 trillion over 10 years. “How will she pay for it all?” the GOP asks.

But making the case is complicated for Republican presidential contenders, who have all but abandoned the idea that their tax-cut proposals should be offset by other tax increases or spending cuts.

On a call for reporters organized by the Republican National Committee on Wednesday, officials suggested that Mrs. Clinton’s spending proposals would run up the deficit and create a new era of big government.

“This is dangerous if not reckless management of the federal budget,” GOP economist Doug Holtz-Eakin told reporters.

Other Republicans have picked up the line of attack. “National debt is approaching $19 trillion & @HillaryClinton wants to add $1 trillion in new spending on top of it,” former Florida Gov. Jeb Bush tweeted this week.

There is little dispute about the cost of the Clinton proposals, which include subsidies for college tuition, infrastructure spending and a subsidized child-care program. And Republicans are right that Mrs. Clinton has not yet fully explained how she will offset the cost, though she is promising she will.

A new analysis by the Committee for a Responsible Federal Budget says that the tax increases Mrs. Clinton has already proposed—particularly a plan to limit deductions by the wealthiest taxpayers—come close to $1 trillion, depending on the details, said Loren Adler, research director for the nonpartisan group.

By contrast, most Republican presidential candidates are proposing tax cuts that would cost trillions of dollars without saying whether or how they would make up the lost revenue. Many argue that deficits are less important today than the economic growth they say would come from tax reductions, and that returning taxpayers their own money is not the same as spending on new programs.

“It’s a bit of a litmus test for conservatives: if you think tax cuts should be considered the same as ‘spending,’ then you’re probably a liberal,” said Alex Conant, a spokesman for Sen. Marco Rubio (R., Fla.).

This is a switch from 2012, when GOP nominee Mitt Romney proposed a cut in tax rates but promised it would be offset, for instance, by ending various deductions and cutting federal spending. He stayed committed to that principle, even when he came under pressure to identify those offsets and to show his plan added up.

“My number one principle is there’ll be no tax cut that adds to the deficit,” he said during a fall 2012 debate with President Barack Obama.

This year, the GOP candidates don’t seem concerned with that. “Every single GOP tax plan released so far would add significantly to the debt,” Mr. Adler said.

He said that the tax plans put forth by Mr. Bush, Sen. Ted Cruz (R., Texas) and former Sen. Rick Santorum would all cost the federal government some $3 trillion to $4 trillion over 10 years and that additional taxes generated by stronger economic growth would make up only a fraction of the loss. He said a version of Mr. Rubio’s plan costs some $6 trillion, including a new tax credit for lower-income families. Celebrity businessmanDonald Trump‘s plan is the most expensive, costing some $12 trillion over a decade, he said.

The Clinton campaign is charging hypocrisy. In a blog post Wednesday, the campaign said that the Clinton plan “will be fully paid for, while the Republican plans … will add trillions of dollars to the debt.”

Mr. Holtz-Eakin said he would like more detail from the GOP field as to how their tax cuts would be funded. “I do think any candidate has an obligation to show how it adds up,” he said.