the role of an opposition party on the right is both to oppose the ideas presented by the left and to continually propose new ideas of their own. it does not expect to govern, either and is generally representative of minority interests in society - be they christian, muslim or whatever else. however, it will govern from time to time as the ruling party needs a break from government, a role it is better suited for than the left party because it is fundamentally conservative in nature, and will hold to the status quo until another round of synthesis can begin. when left parties take over instead of right parties, it signifies a rapid collapse in the fabric of society, perhaps brought on by the application of capitalist economic doctrines. mitch mcconnell actually does an outstanding job in this role, he's just missing any sort of balance.
the role of the ruling party in the centre - which is potentially the democrats - is to actually write legislation, by looking at all available data, including that presented by the two other parties. it needs to be the serious party, both idealistic in scope and yet grounded firmly in fiscal realities. it needs to actually understand the math, because it's writing the rules. as it will adopt a quasi-judicial role, and it is writing actual legislation, it needs to also explicitly rely on the judiciary for interpretation whenever possible.
it is only by separating the three parties into these specific roles that the aspirations of young politicians can be kept in check enough to prevent them from being captured. even so, a flow of capable policy wonks would be expected to migrate from both the left party and the right party to the ruling party in a fairly regular stream, and this should be seen as something to slow down rather than as something to prevent.