If it had been all along - A bit better for people in the US, it's easier for decent causes to slip through the cracks when there are more parties. We'd probably have universal health care and an underfunded national public housing program instead of none. I don't think the MIC would've taken as deep of control of a parliamentary system either, so there'd be fewer omnicidal wars based on selling weapons and destabilizing rivals. Of course the US is still a bloodthirsty place, so those wars would have to be replaced with different ones, probably more colonial resource extraction like France does. But statehood movements would also be more powerful, so a maybe those colonies would become states (which I don't think would temper the bloodthirst, they'd eventually catch the settler colonial mind virus).

If it suddenly transformed at the start of an election cycle - Absolute clusterfuck. The two party system doesn't keep the bourgeoisie in control, that's done by secondary power structures that grew up alongside the two party system to adapt it to the conclusions required by a capitalist system. Those structures wouldn't be able to adapt quickly enough, so we'd have an election where the bourgeoisie parties get way fewer votes than they're supposed to, and popular parties get more. 20% of parliament would go to various competing strains of open fascism, a bunch would go to parties representing competing bourgeoisie factions, and a huge part would go to very generic socdem parties. I think a huge slate of social safety network legislation would pass (and probably some imperialism). It would take a few election cycles for corporations to take over the most popular socdem and fascist parties and start repealing all the social safety nets.