ROME—Matteo Salvini, having established himself as Italy’s leading populist firebrand, is now making political waves in Europe.
The leader of the anti-immigration League party and interior minister is blocking migrant-laden boats from landing in Italy, challenging European Union rules on asylum, and adding to the pressure on Germany’s beleaguered chancellor, Angela Merkel.
Mr. Salvini, 45 years old, is emerging as a key player in a network of European nationalist politicians who want a tougher immigration crackdown. His growing clout in Rome, and Italy’s role as a gateway to Europe for migration from Africa, are making him a dangerous adversary for Ms. Merkel as she struggles to work out a common EU approach on migration.
Immigration policy is the central battleground in Europe’s deepening political divide. A centrist, pro-European Union establishment is seeking to reassure voters that cooperative measures can curb migration flows while spreading the burden of taking in refugees fairly. Antiestablishment political insurgents, particularly on the far-right, are denouncing EU efforts as a failure and seeking to sweep away longtime incumbents such as Ms. Merkel.
European populists have been in close contact to develop common positions. Some politicians in Italy, Austria and Germany have called for an “axis of the willing” to combat illegal immigration.
“Italy has stopped bowing its head and obeying,” Mr. Salvini said this month after blocking a ship bearing asylum seekers from landing. He has long accused Italy’s centrist politicians of doing what Germany and EU authorities wanted, including by continuing to take in asylum seekers from Africa, who are often brought across the Mediterranean on rescue ships after taking to sea in rickety boats.
His rhetoric has shocked many Italians, drawing accusations of xenophobia. On June 18, he called for counting and registering the Roma minority, while deporting some. “Unfortunately we will have to keep the Italian Roma,” he told a local television station.
Since 2011, more than 750,000 refugees and other migrants have reached Italy after being rescued in the Mediterranean. EU law holds that asylum-seekers are entitled to apply for international protection once they reach the mainland.
The number of arrivals has fallen sharply in the past year as Libya, with Italian encouragement, has cracked down on movement through its territory. Only about 16,500 people have landed in Italy this year. However, the potency of the migration debate continues to grow in Italian politics.
When EU leaders try to find answers to Europe’s migration problems at a Brussels summit on Thursday, Italy will be represented by Prime Minister Giuseppe Conte. But Mr. Salvini and his League are the real power behind the new government in Rome, overshadowing their more moderate partners, the 5 Star Movement.
The League’s popularity continues to grow, reaching around 28% in the latest opinion polls, compared with 17% in March elections.
Mr. Salvini’s provocative style carries strong echoes of U.S. President Donald Trump, from his prolific social-media posts to his “Italians First” campaign slogan.
His first major foray into European politics came last week, when he said Italy wouldn’t agree take back migrants from Germany to help relieve pressure on Ms. Merkel. “The Italian government is willing to help only Italians,” he said.
Ms. Merkel has been facing intense pressure from Bavarian conservatives in her ruling coalition who want to close Germany’s borders to asylum seekers already registered in another EU country.
The German leader, who defends open borders within Europe, is pushing for voluntary arrangements under which asylum seekers would be sent back to countries such as Italy where they first landed in Europe. The rift in Ms. Merkel’s government over the issue is so deep as Berlin is rife with speculation the chancellor could fall.
Mr. Salvini, however, is demanding that the rest of Europe take more migrants off Italy’s hands—rather than sending some back. “Instead of being willing to take, we are willing to give,” he said.
“The destiny of Merkel hangs partly on how much Italy is willing to concede,” said Federico Santi, an analyst at political risk consultancy Eurasia Group. “The problem is greater for Germany than for Italy. Salvini is gaining, while Merkel sees her position under threat.”
The deepening dispute over how to distribute migrants within the EU is leading European governments to focus on blocking off migration routes within Africa—one thing on which Mr. Salvini and Ms. Merkel agree.
On Monday, Mr. Salvini visited Libyan leaders and promised more Italian vessels and training for the Libyan coast guard. Italy wants Libya to patrol its coast and stop migrant-laden boats from heading to Europe. Mr. Salvini also called for EU-financed migrant-processing centers in African countries to Libya’s south.
“The priority for Italy is to protect [Europe’s] external borders. We must solve the problem at its roots,” he said.
Write to Giovanni Legorano at giovanni.legorano@wsj.com