The liberal SDP split with the communists, supporting "centrist" Hindenburg in the name of unity.
The communists campaigned on "A vote for Hindenburg is a vote for Hitler is a vote for war"
Hindenburg won the election, getting more votes than either the communist or fascist candidates.
Hindenburg, the liberal candidate, then proceeded to make Hitler the chancellor and staff positions of power with nazis while purging the government of communists.
The nazis then barely had to do anything to assert complete control.
The nazis didn't get in power because communists stayed home, they got into power because the liberals would rather work with fascists than communists.