The man who is favored to be Germany’s next chancellor has opened the door to working with the Alternative for Germany to pass tough new immigration restrictions, potentially breaking a longstanding effort to shun a party whose flirtation with Nazi language has made it anathema to the political mainstream.

The opening by Friedrich Merz, the leader of the center-right Christian Democrats, who leads in the polls for the chancellor election next month, came after a knife attack last week in Bavaria by a mentally ill Afghan immigrant that killed two people, including a toddler.

The attack, the latest in a string of high-profile killings carried out by immigrants, has since upended Germany’s presidential election, set for Feb. 23, refocusing what had been an economy-themed campaign toward the contentious issue of migration.

Mr. Merz is trying to show voters that he and his party are serious about tightening Germany’s borders and following through on deportations of migrants whom authorities have determined should leave the country.

But until now, all parties at the national level had built what is colloquially known as a “firewall” around the AfD, hoping to blunt the party’s move into the mainstream.

The AfD is currently running second in polls before the election, sitting comfortably ahead of Chancellor Olaf Scholz’s Social Democrats, though well behind Mr. Merz’s Christian Democrats.