Fears that the government of President Emmanuel Macron of France may collapse for a second time in nine months amid a looming debt crisis stoked alarm in financial markets on Tuesday. Investors hammered the French stock market and pushed up the country’s sovereign borrowing costs to among the highest in the eurozone.

Adding to the concerns was a warning on Tuesday by the French finance minister, Eric Lombard, that France might need assistance from the International Monetary Fund if the crisis could not be brought under control. “I cannot assure you that the risk of I.M.F. intervention does not exist,” he said in an interview on French radio.

France, a cornerstone economy in Europe, is rapidly becoming one of its weakest links. As Mr. Macron has been playing the global statesman — sitting next to President Trump to push a European deal for Ukraine or declaring France’s recognition of a Palestinian state — the financial situation back home has been falling apart.

The turmoil mounted after Mr. Macron’s prime minister, Francois Bayrou, issued a surprise call on Monday for a parliamentary vote of confidence on Sept. 8 to address the “gravity” of the problem. France’s debt and deficit have ballooned virtually unchecked to one of highest levels in Europe.

French opposition parties doubled down on Tuesday on threats to topple the government, vowing to vote against a budget that Mr. Bayrou will present at the Sept. 8 session to slash 44 billion euros from the deficit immediately. Part of the budget involves cutting two national holidays, which has caused nationwide fury.

The fall of France’s current government is highly likely and would weigh heavily on the country’s economy, ING Bank said in a note. The economy was already weak, with growth forecast at just 0.8 percent this year. The political crisis “adds a new layer of uncertainty,” wrote Charlotte de Montpellier, the bank’s senior economist for France.