BERLIN (Reuters) - Germany’s highest court ruled on Tuesday that a former cabinet minister had breached neutrality rules that apply to government members when she accused members of the far-right Alternative for Germany (AfD) of promoting radicalization.

The ruling underscores the challenges facing politicians trying to take on the AfD, which won nearly 13 percent in a Sept. 24 election at the expense of Chancellor Angela Merkel’s conservatives and the center-left Social Democrats (SPD).

In 2015, at the height of a large influx of Muslim migrants, Johanna Wanka, then education minister, responded to an AfD slogan at a Berlin protest rally that read: “Red card for Merkel - asylum needs borders.”

Wanka issued a statement on the ministry’s website saying the red card should be shown to the AfD, not Merkel, adding:

“Bjoern Hoecke and other party spokespeople are contributing to social radicalization. Right-wing extremists who openly incite hatred and violence, like the head of (anti-Islam grassroots movement) PEGIDA boss Lutz Bachmann, thus receive intolerable support.”

Hoecke, a senior member of the AfD, has described the Holocaust memorial in Berlin as a “monument of shame” and wants German history books on the Nazi era to be rewritten.

“EQUAL OPPORTUNITIES”

The Constitutional Court said on Tuesday that Wanka had “violated the AfD’s right to equal opportunities of political parties” as her remarks may have negatively affected voters’ view of the party.

While the government is allowed to explain its actions in response to criticism, it said, state organs are not allowed to react to “unobjective and defamatory attacks” in the same way.

AfD leaders welcomed the decision, saying it was a clear signal to the government not to misuse taxpayers’ money for “political agitation against the opposition”.

“Thank goodness there are judges in Karlsruhe,” said AfD leader Alexander Gauland, referring to the southwestern city where the court is based.

Since the election, the AfD - originally set up in 2013 as an anti-euro party but which has morphed into a nationalist party opposed to immigration - has gained in polls. An INSA poll on Tuesday put the AfD on 16 percent, just ahead of the SPD.

The main parties are trying to counter the AfD’s positions with facts and arguments, but checks and balances embedded in Germany’s political system since World War Two mean they have to respect the AfD’s legitimacy in parliament.

The AfD will become the main opposition party if SPD rank-and-file members, voting in a postal ballot, endorse a new coalition government between their party and Merkel’s conservatives.

The mainstream parties have always said they would not enter a coalition with the AfD. On Tuesday the conservative bloc agreed unanimously not to work on or to submit motions together with the AfD during this legislative period.

The AfD will be entitled to head some parliamentary committees, including the powerful budget committee.