I'm pretty sure in this hypothetical situation the US 2 party system would have to be opened up.
Point taken though. Lets not fall into the trap of "you are either for or against x", I'm sure a very large proportion of Liberals prefer the Tories to the NDP.
It was a big deal when Alberta wanted to
legalise private health care. Opinion polls consistently put Canadians at about 80/20 Democrat. Adding Canada onto the USA would guarantee Democratic victories until the parties had a chance to realign.
After the political centre shifted left, you'd eventually see the GOP shift left as well and some swing states like Virginia, North Carolina and Colorado swing back to the GOP as former NDP politicians make the Democrats the wacko party.