As recently as a couple of months ago, the U.S. dollar was the woozy heavyweight of global foreign-exchange markets, undergoing periodic swoons amid speculation that the euro was threatening the dollar's status as the world's dominant currency. Casual comments by foreign central bank officials hinting at their diminished appetite for dollars triggered massive sales of greenbacks by traders around the globe.

Now it's the euro that is suffering sharp depreciation at the hands of jittery investors and loose-lipped officials in the aftermath of the rejection this week by French and Dutch voters of the proposed European constitution. Not only is the euro down more than 10 percent from its high at the end of 2004; its guardians have been forced to rebut speculation that the lack of cohesion in Europe might imperil the euro's very existence as the common currency of 12 countries.

The latest sign of the role reversal between the two currencies came yesterday. Despite a disappointing government report on job growth in the United States that would ordinarily have driven the dollar lower, the euro ended the day trading at $1.2236, down from $1.2267 the day before, capping a 2.9 percent decline for the week. Among the reasons for yesterday's slide in the euro was the publication in an Italian newspaper of comments by Roberto Maroni, the country's labor minister, who said that reintroducing the lira was "not such a crazy idea."

That Maroni belongs to the smallest party in the governing coalition and that labor ministers don't set currency policy helped moderate the impact of his remarks. But the statement came atop other speculation about weakening official support for the euro, including a report in the German magazine Stern (later denied) that top German economic policymakers had discussed the possibility of European monetary disintegration. And economists, commentators and market players are suddenly finding it irresistible to conjure up scenarios in which some or all of the countries that adopted the euro might abandon it.

In a report to investors this week, Ray Attrill, the London-based research director for advisory firm 4CAST, wrote, "The question of how to position for the 'low probability, high impact' event risk of [European Monetary Union] break-up . . . is now being asked with a little more seriousness."

Small wonder, therefore, that talk has receded of a dollar crash in which foreign investors and central banks flee the currency in favor of the euro. But such risks have by no means disappeared, and the dynamics that are causing the euro to lose altitude may not last long, analysts warn.

"People are completely on the other side of the ship at the moment" in their assessment of the dollar's strength relative to the euro, said David Gilmore, an economist at FX Analytics in Essex, Conn. "The emphasis is on 'moment.' The things that got the euro so high against the dollar at the end of December -- the huge [U.S. trade and budget] deficits -- haven't gone away." As for the European Union's political troubles, Gilmore added, "The history of any country forming a federation of states and constitution usually isn't pretty -- the U.S. included."

Talk of the euro's demise drew widespread scorn from many European officials and economists alike. "Totally absurd," European Central Bank President Jean-Claude Trichet said yesterday, likening the idea to California or Texas or Alaska deciding to have their own currencies.

Not that the euro isn't under plenty of strain. Some countries that use the currency, such as Spain and Ireland, are booming, while others, such as Germany and Italy, are stagnating. That is raising serious tensions within the euro zone about whether the central bank is keeping interest rates too high or too low. The failure to approve the new constitution only increases the chances that individual countries' economic policies will diverge, along with their growth rates.

But withdrawing from the monetary union would be "economic suicide" for countries such as Italy, as Otmar Issing, the ECB's chief economist, put it yesterday. Joining the euro zone, after all, caused Italian interest rates to decline sharply, as investors no longer had to worry much about the possibility of inflation in Italy. Abandoning the euro would almost surely cause Italian rates to jump.

"Traditionally, when the European integration process gets hit by a blow like this, they regroup and usually use it as a spur for progress," said C. Fred Bergsten, director of the Institute for International Economics.

The situation may be more worrisome, in the long term, for the dollar than for the euro, Bergsten added. The recent rise in the dollar will worsen the U.S. trade deficit by making American exports more expensive, he noted, so "it would not lessen my concern" about a possible dollar collapse. "In fact, it intensifies it."