How could the Socialist Party of the early 1900s become a major political force in the Rust Belt (Michigan, Ohio, Indiana, Pennsylvania)? Any POD is acceptable.
I can think of several different PODs. I'll go through them chronologically.
1. McKinley isn't assassinated, keeping TR's pragmatic progressivism out of the White House, dwindling the chances for reform within the two major parties.
2. At the SP convention in 1912, have the left win control of the national executive, so that the Socialist Party follows the German model of party development.
3. Have the US remain neutral in WWI, thus negating the Red Scare. Socialist pick up a foothold in the House in 1918-20.
4. If WWI does occur, keep the Left-Wing from being expelled in 1919. The party organization, though beaten by the Red Scare, remains in tact, and picks up a foothold in the House when demobilization goes sour.
5. Have the SP's cooperation with the AFL in LaFollette's 1924 campaign continue, leading to the AFL's official endorsement. Socialist Party starts kicking northern Democrats out of their seats.
6. Failing all of that, have business as usual continue during the Great Depression. A conservative Dem wins in 1932, but conditions fail to improve sufficiently, and the government appears to be doing nothing. The Socialist Party has a decent shot in the 36 election and after.
7. Finally, have the CIO go left, and endorse a Socialist/Communist popular front in the 30s. Find a way to stop Taft-Hartley or things of the kind, and the coalition could stick.
In basically all of the scenarios, the SP only will become a major player after the start of the Great Depression. And, at any rate, given America's electoral system you're more than likely going to kill one of the major parties. Which one it is will depend on how the Republicans respond. They will have to work to beat the Democrats on their own turf in the South, or once the Socialist Party takes the Rust Belt and New York, it will be game over for the Republican Party