BERLIN — In November, as Covid-19 cases began to rise, thousands of people gathered in Berlin to protest against restrictions. In among the conspiracy theorists and extremists were several lawmakers from the country’s main opposition party, the far-right, anti-immigration Alternative for Germany.

It was striking to see legislators mingle with conspiracists in the streets before heading to the parliament for a debate. Yet it wasn’t too surprising. The party, known as AfD, has sought to improve its electoral standing ahead of the national election in September by associating with the anti-lockdown movement, an amorphous mix of conspiracy theorists, shady organizations and outraged citizens.

But it hasn’t worked. In the months since the pandemic, the AfD’s support has slipped. Already struggling to reach new voters, its embrace of anti-lockdown sentiment seems to have further limited its appeal — and sped up its transformation into an extremist organization.

When the pandemic reached Germany in March, the AfD’s initial response was cautious. Prominent party legislators warned about the virus, encouraged the government to act swiftly and voted for a package of economic relief. “Closing ranks is our first duty as citizens now,” Alexander Gauland, a co-leader of the party, said.

But this attempt to cater to the average voter came at a cost. The party soon found itself deprived of many of its usual supporters, who took a different course, downplaying the danger and castigating the government. On Facebook and social media, the party stuttered. “The AfD,” said Johannes Hillje, a political consultant who analyzed the party’s social media performance during the pandemic, “lost its rage machine.”

For a party fueled by indignation, that was a problem. As the first lockdown was tentatively lifted, through April and May, many leading AfD figures performed a 180-degree turn. No longer consensual, they fiercely railed against restrictions of any kind, which they claimed were unconstitutional as well as economically ruinous.

In November, to demonstrate its defiance, the party held an in-person convention with hundreds of participants packed into a hall. That same month, an AfD legislator appeared in the parliament, where masks are mandatory, wearing one riddled with holes. And prominent party members not only attended some of the anti-lockdown protests that spread across the country last year but also adopted the protesters’ talking points, for example by calling Germany a “Corona dictatorship.” The AfD became something like the anti-lockdown party.

The move made sense. By the time the pandemic arrived, the party “had started to struggle,” Kai Arzheimer, a professor of political science at the University of Mainz, told me. Migration had vanished from the top of voters’ concerns, depriving the party of its momentum. It was unclear how it might make further inroads.

What’s more, the party was increasingly seen as extreme and radical. The media uncovered many ties to extremist groups such as the Identitarian Movement, which advocates ethnically homogeneous societies, while a radical internal group gained power. The AfD was considered so dangerous that the domestic intelligence service even put one wing of it under surveillance. “This has harmed the party’s potential to mobilize moderate voters,” Mr. Arzheimer said.

Unable to appeal to more moderate voters, and in the midst of a pandemic that shored up support for the major parties, the party entwined itself with anti-lockdown radicalism. By conventional measures, the move has failed. National polls routinely place the party at or under 10 percent approval; two regional elections this Sunday are expected to underline the party’s electoral difficulties. The historic showing of 2017 — when the AfD became the first far-right party to enter Germany’s postwar parliament — is unlikely to be repeated, let alone surpassed.

That doesn’t make the party less of a danger, though. In ways reminiscent of former President Donald Trump, the AfD is seeking to scuttle public trust in the political system. An AfD legislator suggested from the floor of the parliament that mail-in ballots were one of many “dark ideas” with which the other parties hoped to rig the vote, while a section of the party has run ads on Facebook warning against the practice.

Ahead of an election where many may vote remotely — Germany’s vaccination program probably won’t be complete by fall — this amounts to a calculated strategy of subversion. Though the party’s influence is limited, the fact that 8 percent to 10 percent of the electorate seems unshakable in its support is deeply concerning.

In a landmark decision last week, the country’s domestic intelligence agency put the entire AfD under surveillance, branding it an extremist organization. Whether it’s right to do so — and whether the order, which was suspended and is under legal challenge, will be enacted — is hard to know. But the AfD, and the danger it potentially poses to Germany’s democracy, is not going anywhere.

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