MILAN — One by one on Saturday, Europe’s far-right politicians took turns from the podium in a thronged central piazza, describing the threat of Islam and decrying the runaway decadence of the Brussels elites.

They hugged and took selfies with Italy’s Matteo Salvini.

“We loved it,” said Geert Wilders, the leader of the Dutch Party for Freedom.

The rally amounted to a chest-puffing show of force from Europe’s nationalists, who have upended politics across much of the continent. They now want to seize influence in Brussels and weaken the European Union from within.

Twelve far-right parties have formed an alliance — a “patriot family,” some called it — ahead of next week’s European Parliament elections. Salvini, the head of Italy’s nationalist League party, is its headliner.

But for all the gains of right-wing populism in recent years, the group is now meeting perhaps its greatest challenge — testing whether parties that have vowed to put their own country’s interest first can work together and alter the European agenda.

There are reasons to be skeptical.

Though polls project that anti-establishment parties might win as much as 35 percent of the vote for European Parliament seats, those parties reflect varying strands of populism and nationalism. They also have formed sometimes-competing European alliances.

Salvini’s group — though bigger than far-right groupings in previous European elections — encompasses only some of the continent’s insurgents and renegades, not the full lot. One of Salvini’s allies, France’s Marine Le Pen, told reporters Saturday that the group on display in Milan might become the second- or third-largest force in the European Parliament.

Polls, though, suggest it will win about one-tenth of parliamentary seats, finishing fourth — making it an alternative to the center-left and center-right. Its success is hindered because two of Europe’s most significant nationalist parties, from Poland and Hungary, have yet to join forces with Salvini’s group.

Even among those parties that have banded together, major policy differences remain.

“[Far-right parties] have got an agreement about this vague idea of a Europe of sovereign nations,” said Susi Dennison, a senior policy fellow at the European Council on Foreign Relations. “But underneath those headlines, there are huge differences.”

Those differences touch on some of the deepest problems on a continent facing sluggish economic growth, an unresolved political battle over migration, stark geographical disparities, and shifting alliances with global powers.

Far-right parties from the more prosperous north say the European Union should pare back its budget. But nationalist hotbeds of Poland and Hungary depend disproportionately on funding from Brussels and don’t want it to be curbed.

Meanwhile, far-right parties along the Mediterranean want member states to do far more to share the burden of accepting migrants, but other nationalist governments have fully closed their doors, refusing to take part in E.U. resettlement programs.

And far-right parties in Eastern Europe are perpetually on guard against Russia. Salvini, however, has flown frequently to Moscow, called sanctions against Russia “absurd,” and once wore a Vladimir Putin T-shirt at an E.U. parliamentary session.

Even at the rally Saturday, it was clear that the goals in Europe were second to those at home in Italy.

Salvini — the Italian government’s interior minister — has used a mix of charisma, anti-migrant policymaking and inveterate campaigning to become the country’s most influential politician.

People in the crowd in Milan said they had barely a clue about the other politicians. They were there to see one man. The stage was decorated with a half-dozen logos of Salvini's party, the League, with no reference to other European parties or flags.

“Italy First,” a sign on the podium said, and most of the politicians standing behind that podium received only modest applause — unless they mentioned Salvini.

“Thank you, Matteo,” one politician after the next said in concluding their speeches.

“Europe needs more Salvinis,” Wilders said onstage to big cheers.

In an interview, Wilders, who has called Islam a “medieval cult” and said Europe is facing “cultural suicide,” acknowledged differences with other far-right parties, but he said they did not need to vote as a bloc in Brussels, so long as they could cooperate on the biggest issues.

“Let us accept our differences,” Wilders said. “Let us vote differently. At the end of the day we have more in common than we have differences.”

In recent months, Salvini traveled to both Poland and Hungary, trying to marshal the right wing forces. Stephen K. Bannon, the former Trump chief strategist, launched his own unite-the-right campaign in Europe — though he was thwarted in some countries by some national electoral laws banning foreign funding.

Analysts have speculated Hungarian leader Viktor Orban might abandon his current European group and jump to the far-right alliance. But he would have to overcome some personal wariness.

In a recent interview with author Bernard-Henri Levy, Orban lauded Salvini but said, without explaining himself, that he saw an alliance with Le Pen as a “red line.”

Orban’s Fidesz party, for now, remains a part of the traditional center-right force in Brussels, known as the European People’s Party, or EPP. But Orban, with his defiance of bloc norms, has long been a headache for the Brussels centrists — and Fidesz was suspended from the EPP earlier this year.

The full influence of the far-right in the next European government remains to be seen. But nationalists will get some seats in Europe’s de facto cabinet, the European Commission.

They will also be able to agitate for changes to migration and economic policy — at least in moments where they agree on what to say. Perhaps most significant, more nationalists will have an insider’s seat in Brussels — and can return to their voters with stories about the institution’s perceived excesses or failures.

“They can demonstrate that Europe is incapable of acting, that it doesn’t have a good answer — on dumping from the Chinese, on migration,” said Mujtaba Rahman, the Eurasia Group’s managing director for Europe. “That feeds their own narrative and feeds their own agenda — the Orban and Salvini idea of erosion from within.”

Salvini, in his speech Saturday, did not describe the particulars of his plans for Europe. But he termed the far right’s battle as one of David against Goliath, an effort to push back against the superstate powers.

“The small, when they are motivated, defeat large powers that are arrogant,” he said.

The crowd chanted his name: “Matteo! Matteo!”

As he spoke, the gloomy weather improved. Some in the crowd folded up their umbrellas, something that Salvini read as a sign of the group’s power.

“We even made the rain stop,” he said.

Stefano Pitrelli contributed to this report.