All it took was an all-caps social media threat by President Trump to impose a 200 percent tariff on European wine for the shipments of many Brunellos, Chiantis and Proseccos to come to a shuddering halt.

In Tuscany, Italy’s most famous wine-exporting region, thousands of bottles meant for American tables are stranded in the wineries’ chilly cellars or in storage rooms in Livorno, the port city from which they were to depart.

“Everything is stopped,” said Tiziana Mazzetti, the sales and marketing manager of the Old Cellar, a winery in the Tuscan town of Montepulciano, as she stood among boxes of wine bottles that were supposed to leave this month for the United States. “The damage is already here.”

So far, Mr. Trump’s threat remains just that. But it has been enough for jittery American importers to pause orders rather than potentially pay tariffs that could make the wine unaffordable for some and just not worth it for others. If the tariffs were to be imposed — and passed on to consumers in full — a $20 bottle would suddenly cost $60.

Together with France and Spain, Italy is among the most exposed in Europe to American tariffs on wine, and many say a 200 percent tariff would be devastating. For nearly 15 years, the United States has been Italy’s biggest export market for wines. About a quarter of Italy’s wine exports, or about $2 billion worth, get shipped to America each year.