The national debt would increase, perhaps dramatically, under the tax and spending plans of the leading Republican presidential candidates, according to an independent budget watchdog group.

Leading the way with the most expensive plan is former House speaker Newt Gingrich, followed by former Pennsylvania sennator Rick Santorum and former Massachusetts governor Mitt Romney, says the Committee for a Responsible Federal Budget.

Only Rep. Ron Paul of Texas would keep the national debt in check as a percentage of the economy, the group found, because he has proposed by far the largest spending cuts.

"None of the candidates ever balance the budget," said the group's president, Maya MacGuineas.

The organization assumes the worst in red ink just to get started -- that former President George W. Bush's tax cuts will remain in place, for instance, and that the next round of spending cuts promised by President Obama and Congress will be rescinded.

Even with those debt-increasing measures and others, its analysis found that Gingrich, Santorum and Romney would boost the debt further, mostly to enact even broader tax cuts.

The portion of the nation's total $15.3 trillion debt held by the public, now about 70% of the economy, would rise to 85% in 2021 under the group's pessimistic baseline. Romney would boost that to 96% of the economy because of an expensive tax-cut plan he unveiled Wednesday. Gingrich and Santorum would raise the public debt well above 100%.

In Gingrich's case, the main drivers of the debt would be his plans to create an alternative flat tax of 15%, eliminate all capital gains taxes, drop the corporate tax rate from 35% to 12.5% and establish private accounts for Social Security. His plan would add another $7 trillion to the debt over the course of his potential two terms, the analysis found.

Santorum would greatly reduce individual and corporate income taxes. He promises to cut spending by $5 trillion over five years, but he doesn't say how he would do it.

Romney, whose plan was found the most credible because he offsets many of his tax cuts, would reduce individual tax rates by 20% and eliminate the alternative minimum tax.

"All of these candidates fail," said Alice Rivlin, former director of the White House budget office and the Congressional Budget Office. "They all reduce the revenue available to the U.S. government over time."