As lawmakers struggle to resolve the debt crisis, a growing number of observers wonder whether President Obama has one last trump card at his disposal: ignoring the debt ceiling altogether.

Top Democrats are reviving an argument — one that has arisen several times — that the White House could invoke the 14th Amendment of the Constitution to raise the debt ceiling without congressional approval.

“Is there anything that prohibits him from doing that?” Sen. Tom Harkin (D-Iowa) told the newspaper The Hill. “The answer is no.”

House Minority Whip Steny H. Hoyer (D-Md.) has described it as the least bad option if Congress doesn’t act.

The White House, for its part, continues to resist the speculation.

“Only Congress can increase the statutory debt ceiling,” White House press secretary Jay Carney told reporters Friday. “That’s just a reality.”

But many legal scholars are suggesting that Obama could do it.

Jack Balkin, a law professor at Yale, has laid out how this would work. At some point after Tuesday’s deadline, Obama would face the demands of multiple contradicting laws. On the one hand, the government is required to pay out money that has already been appropriated. On the other, it would not be allowed to float new debt to cover its obligations.

So, Balkin notes, Obama “has a constitutional duty to treat at least one of the laws as unconstitutional as applied to the current circumstances.” And the wording of Section 4 of the 14th Amendment suggests that the debt ceiling would have to give way: “The validity of the public debt of the United States, authorized by law . . . shall not be questioned.”

If Obama interpreted that clause to mean that the debt ceiling is unconstitutional and authorized the Treasury Department to begin issuing new debt, it’s not clear that anyone could stop him. As Jeffrey Rosen writes in the New Republic, individual members of Congress would not have standing to sue — Congress would need to pass a joint resolution, which is unlikely given Democratic control of the Senate.

It is also unlikely that individual taxpayers or bondholders would have standing.

“The most likely outcome is that the Supreme Court would refuse to hear the case,” Rosen argues. And if a suit did make it through, Rosen adds, even the conservative justices would probably rule in Obama’s favor — at least if they held to their judicial philosophies.

But Obama would still face political blow-back. The decision would probably set off an extensive legal and public-relations battle over the scope of the president’s powers. Democrats and Republicans alike were upset about Obama’s decision to intervene in the armed conflict in Libya without Congress’s consent. An unprecedented constitutional maneuver would allow the opposition to paint a portrait of a president who thinks his authority has no bounds.

“I think it is one of those too-clever ideas that, when you reflect on its ramifications, does a lot of bad things,” said Dan Hazelwood, a Republican pollster.

The biggest problem, however, is the practical one — how markets would react to the 14th Amendment option.

“On the one hand it shows that the feared disruptions from no debt-ceiling increase wouldn’t happen,” Tom Gallagher, a fiscal- and monetary-policy specialist at the Scowcroft Group, said via e-mail. “On the other hand it plunges the U.S. into a genuine constitutional crisis. How quickly would courts rule? Who would bring suit? Would the House initiate impeachment proceedings? That’s hardly a reassuring set of questions.”

Nor is it clear that invoking the Constitution would calm the credit-rating agencies. Standard & Poor’s has declared that it will downgrade the nation’s credit rating within the next three months “if we conclude that Congress and the Administration have not achieved a credible solution to the rising U.S. government debt burden and are not likely to achieve one in the foreseeable future.”