In any nation with first past the post elections, like the United States, Leftists have exactly one rational voting strategy:

Step 1. Identify the two front-runner parties, and determine which of the two is further left relative to the other.

Step 2. Vote for that party in every single election (don't forget midterms and local elections). Encourage everyone you know to do the same.

Step 3. Once the (relative) left party has an overwhelming majority (over 2/3) and the relative right party becomes vanishingly irrelevant (under 1/3), then split the (relative) left party into its own relative left and right.

Step 4. Repeat steps 1-3 with these new front-runner parties.

Step 5. Iterate step 4 until your relative left party passes election reform such that elections are no longer susceptible to Duverger's Law.

Certainly try to push for reform within the relative left party between elections and during primaries, but at the ballot box the above is the only rational strategy. Voting third party, or refusing to vote the lesser evil, is not a rational strategy.