Simeon II effectively killed monarchist cause by stepping to politics. It is something what royals shouldn't do in modern era. There is reason why royals even don't vote on elections.
Even in the UK the unpartial monarch was dragged into politics when the PM wanted to suspend Parliament for weeks on a most discutable base. I am sure (the advisers of) Queen Elizabeth knew the monarch was "used" but in the highly polarized climate there was little the palace could do without becoming partisan. In the end the UK Government had to back down due to an unanimous Supreme Court destroying said suspension.
We have seen in Luxembourg what happened when a hereditary head of state, without a democratic mandate, wanted to block legislation which was approved by a democratically elected Parliament after a constitutionally correct procedure: the Grand-Duke was removed from the legislative process. Rücksichtlos. With unanimous (!) vote.
When in Belgium King Albert II had similar problems with a Bill, like his brother King Baudouin had, the then Prime Minister Jean-Luc Dehaene then visited Laeken Castle and sharply rebuked him: "Sire, you are not your brother", with that warning him Parliament would not tolerate a second Baudouin to frustrate legislative process.
That is also why it was unwise of Prince Emanuele Filiberto to run under the banner of the UDC (Christian-Democrats) for a seat in the European Parliament. Later he tried to conquer a seat under Silvio Berlusconi's Italia Libertá. That is openly choosing a partisan position in politics. Never wise for any royal, reigning or non reigning. (But Emanuele Filiberto stated when he was born Italy was already a republic. For him the republic was a fact.)
In Italy the current Duke of Aosta and his son Prince Aimone proudly served in the Navy of the Repubblica Italiana. But that is not politics, that is military conscription and serving your country. The same with the current Comte de Paris and his brother Prince Eudes. Both served in the French army.
The previous Duke of Parma gambled on the banner of Carlism. With doing so he could ever forget the throne of Spain for the Carlist cause. Also here a "unpartial and binding" royal became partisan and did lead a (quite militant) party, destroying his prospects, his wealthy wife's fortune and his prestigious royal marriage.