ROME — Matteo Salvini, until recently, had been this continent’s fastest-rising politician. He was the torchbearer for Trump-style nationalism in Western Europe. His own party, with the pledge to put “Italians first,” was triumphing in parts of the country long dominated by the left. Salvini believed his time in power was only beginning.

“We’ll govern for the next 30 years,” he had said.

Now, Salvini appears to have prematurely squandered his hold on the government — while jeopardizing the momentum that had helped turn Italy into Western Europe’s far-right stronghold.

The abrupt change stems from what is widely viewed as a major miscalculation, one in which Salvini attempted this month to crash the existing governing coalition that included his League, force new elections and become prime minister. But he failed to anticipate what happened instead: that two rival parties would attempt to pause their years-long conflict; join hands; and try to form a new coalition, with the goal of forestalling elections and thwarting Salvini’s ascent.

It is far too early to count Salvini out. He heads the country’s most popular party. He can burnish his anti-establishment credentials in opposition.

But if the new coalition between the anti-establishment Five Star Movement and the center-left Democratic Party takes form, it would leave the League frozen out of power, and force Salvini to take the role of other nationalists in Western Europe: acting as a disruptive voice from the sidelines rather than as a rules-busting, policymaking force from within.

Salvini’s potential setback compounds the challenges facing Europe’s band of nationalists, who made modest gains — rather than the hoped-for surge — in European parliamentary elections last May. Several months ago, a scandal knocked a far-right party out of government in Austria. Nationalist governments in Hungary and Poland continue to hold broad powers, but it is Salvini who has worked the hardest to create a more unified far-right bloc. Many of Europe’s far-right parties have pointed to Italy — a founding European Union member — as an example of how nativist, anti-migrant parties can rise from the fringes into the mainstream.

Now, Italy has become an example of a different dynamic, showing how other parties can find ways to contend with far-right forces that they consider dangerous or anti-democratic.

But for some prominent far-right figures, Salvini’s apparent setback represents an opportunity to rally populist forces.

In a phone interview, Stephen K. Bannon, the former chief strategist for President Trump who has tried to build a nationalist “movement” in Europe, said that Salvini had “underestimated the types of deals” his opponents would make to stop him.

Bannon argued that Salvini’s mistake was not “fatal” and that Italians would continue to support him as their next government draws closer to the E.U.

“I think Salvini will only get more powerful,” Bannon said. “He will stand in opposition. For those in Brussels who think they have won this — all the organs of the establishment — I don’t think they have seen anything yet.”

The next government, should it be finalized, would still be headed by Prime Minister Giuseppe Conte. But it would likely work to undo some of Salvini’s signature moves. The Democratic Party has said it wants the next government to be squarely pro-European. It also wants to revoke a security decree that calls for heavy fines for owners and boat captains who bring migrant rescue vessels to Italian ports without authorization. The head of the Five Star Movement, Luigi Di Maio, has signaled that he would prefer to keep Salvini’s immigration policy in place — a point of contention between the new partners.

On at least two occasions when Salvini — in his role of interior minister — closed ports to such boats, Democratic Party politicians ferried into the sea and boarded the vessels in a sign of solidarity.

Salvini had said that other European countries, as well as policymakers in Brussels, were exacerbating the continent’s migration problem by refusing to help Italy share the burden of new arrivals.

“That was the whole approach of what Salvini was doing — to represent migration as a battle against the E.U. and member states,” said Andrew Geddes, director of the Migration Policy Centre at the European University Institute in Florence. “I think now you’ll see a much less conflictual approach.”

Political analysts and researchers who study populism say the League still has deep strength — with a support rating of 34 percent, the highest in the country. It has helped to reshape national attitudes, particularly on migration. If relegated to the opposition, Salvini could sit out the upcoming budget negotiations that could lead to unpopular spending cutbacks. Salvini said last week that life in the opposition would be “likely easier and more comfortable.”

“In some ways, going in and out of government can suit Salvini very well,” said Rosa Balfour, a senior fellow at the German Marshall Fund of the United States. “He can criticize on everything and ride back into office on an anti-establishment vote. The volatility of Italian politics suits a populist like Salvini very well.”

Others are bullish on Salvini’s future for a different reason: They simply think the mash-up between the Five Star Movement and the Democratic Party won’t last very long, even though nearly 80 percent of grass-roots members of Five Star voted Tuesday in favor of the alliance. Both parties were interested in forming a coalition out of concern over performing poorly in a new election.

But Italy also has a history of churning through political personalities. Only a few years ago, Matteo Renzi, of the Democratic Party, waltzed into the prime minister’s office, brash and youthful, with a little populist energy of his own; but he stepped down after losing a 2016 constitutional referendum.

It is also unclear whether Salvini can continue to thoroughly command the public spotlight. Over the 14 months of his League-Five Star Movement government, in which he also served as deputy prime minister, he hooked the country on his ­Facebook and Twitter feeds, with a mix of campaigning and ­migrant-bashing, sometimes mixing in a photo from the beach.

Salvini, so far, has tried to depict the new coalition as a marriage arranged in Brussels — something his allies and others in his party have echoed. The League has announced a protest rally in Rome in October, and Salvini has said that once the government fails, “the word will return to Italians.”

“Nobody can fail to see that the nascent government has no political project,” said Massimiliano Fedriga, a League politician who serves as governor of the Friuli-Venezia Giulia region. “It was born against the League and against Salvini.”

Stefano Pitrelli contributed to this report.