IOTL the Whigs essentially split into two parties, the nativist American Party and the free soil Republican Party.
In the 1856 presidential election, the Democratic candidate, James Buchanan finished first, the Republican candidate Fremont, second, and the American candidate, Fillmore, third and about 20% behind Buchanan in the popular vote. The Republicans attracted ore northern and cross-over Democratic support.
After that the American Party faded from the scene pretty quickly.
Keeping the party platforms pretty much the same, though you can change the candidates, what would it take to make the American Party one of the two major American parties, or even to have them win the 1856 presidential elections? And given the POD or PODs needed, what would be the effects?
Presumably the Democrats would be the other party and the Republicans fade away, but I'm persuadable by other ideas.