Playing as the US, it is 1856 I am always told that "A coalition of Conservative and Reactionary parties took 100% of the vote" despite voters being majority Liberal. Kind of hard to have a civil war when the liberals never get a single vote in each election.
This is a combination of two issues:
-First, POPs vote in Lower House elections by issues and not ideology. So, if a Liberal POP nevertheless favors Interventionism, they'll vote Democrat. Ideology dictates how POPs vote in the Upper House only.
-Second, First Past the Post means that the winning party always wins with 100%. You could have 49.8% of people vote Republican or Whig, and you'll still be told that "A coalition of Conservative and Reactionary parties took 100% of the vote". The other two systems will display actual smaller numbers (though the one that isn't Proportional - Jeffersonian, I think? - will still inflate the winning party's share)