> The most effective single thing to promote a multiparty system is to switch to ranked-choice or approval voting (if staying with single-member districts) or to switch to multi-member districts with some kind of proportional representation.
The single best is to switch to multimember proportional for the legislature (which can remain candidate centric using, say, 5 member districts and STV), and that gets even better (though procedurally more difficult to adopt in the US) if you were to switch the Presidential election from a single winner two-seat President and Vice President to ranked choice two-sequential winner system (e.g., IRV or Bucklin, but after you pick a winner, eliminate that candidate, and tally again without them for a second winner as VP) where each party is structurally incentivized to compete for both spots. These not only make more parties viable they also each offer more election-day choice among candidates of the same party, denying incumbents of favored parties an uncontested sinecure.
The single best is to switch to multimember proportional for the legislature (which can remain candidate centric using, say, 5 member districts and STV), and that gets even better (though procedurally more difficult to adopt in the US) if you were to switch the Presidential election from a single winner two-seat President and Vice President to ranked choice two-sequential winner system (e.g., IRV or Bucklin, but after you pick a winner, eliminate that candidate, and tally again without them for a second winner as VP) where each party is structurally incentivized to compete for both spots. These not only make more parties viable they also each offer more election-day choice among candidates of the same party, denying incumbents of favored parties an uncontested sinecure.