So talking shit about other parties and/or alleging there's a conspiracy to keep them out of power (ie. brandmauer) should be enough to get them barred for being "anti-democratic"? That seems a hilariously low bar and totally arbitrary. It's not exactly an unpopular talking point on the left that politics are being secretly controlled by The Billionaires™ (or Israel). Should they be barred as well?
If it can be shown that left-wing politicians in key positions of their party want to abolish core concepts of the democracy or constitutionally guaranteed rights: sure.