Your moderate party just won an election.?
Xenophobic/populist parties are primarily gaining votes from people who are being hurt by immigration policies. The best way to neuter these parties would be to provide economic programs for these voters. Also beyond the economic issue is the feeling of safety. When you find 1000 German women raped in one night by immigrants that scares the crap out of voters. The German government responded by trying to cover it up. What they should have done is aggressively pursued the rapists and put them in prison. Instead they walk free to rape again and this makes people feel like the government doesnt care because it isnt their daughters getting raped.