Okay, so here's my argument that Social Democracy can be supported with a Buddhist framework. Like those that ground democracy in natural law arguments and divine agency in the west- there are some Buddhist concepts one can ground democracy in.
The need for democracy and a representative government that protects certain reasonable rights for citizens is arguably ideal in Buddhism- firstly because oppression and the suffering it generates crushes people and takes away even their motivation to seek anything better.
Besides suffering being anti-thetical to Buddha- one in oppressive suffering (under tyranny) isn't likely to seek the Dharma, or undertake practice. They'll be too weighed down with concerns and problems- trying to preserve their basic well-being and security against an aggressive power. Like an endlessly hunted animal.
Buddhism is best able to be pursued in a more free society. This is true of any higher knowledge or state a person might seek. Being in better, more optimal conditions opens a chance to inquiry and pursuit of ideals.
This is one reason a Buddhist can and arguably should support democratic government. Creating the conditions for the Dharma's pursuit and practice is a virtue in the path.
Another reason democracy can seem ideal for a Buddhist is the virtues (paramitas). These paramitas are tied with the secret being in many Tibetan teachings, which take their influence from the early Mahayana school of Yogachara.
This goes with a view that the virtues somehow arise from the Dharmakaya (true Dharma body) connected with Ultimate Reality. That when one masters the virtues, they have returned to their fundamental being as Buddha and are part of the work of generating good karmic fields of benefit to all beings, including beings yet to be born. This could be said to constitute something like a Buddhist natural law, for the purposes of legal theory.
The virtues of giving, vitality, knowledge, wisdom, and the like.
These virtues entail certain individual freedoms a person must be guaranteed, just as with natural law theory. That humans can practice these virtues constitute a kind of 'fitness for the rule of law'. That is, we are good beings at the core of our nature, and able to understand justice and expediency in a societal context.
We don't need to be driven like mindless, base beasts under a tyrannical fist- because we aren't. There is in us reflected something higher- the transcendent Buddha-nature. This is just like Christian humanist and enlightenment arguments that humans are fit and equipped for representative government. Nothing better conforms to our natural good qualities and virtues.
The reason I stated that it has to be a Social Democracy though (using Socialism as it's economic ideal) is because Capitalism's glorification of wealth and profit are fundamentally incompatible with Buddhist morality. There is no justifying empowering the greed impulses, when according to Buddhism they are poisons.
Buddhism instead emphasizes cultivating compassion and pity for everything, which leads to ideas about common welfare. Anything that is not useful to bettering this common welfare and only furthers wealth-hoarding is adharma.
I could probably give a more expounded argument for this, but I'm lazy.
