If you're the comrade who finds yourself in this situation, the smart move is not to immediately reveal your power level. You would definitely govern left-wing from an American perspective - do Bernie shit, basically. But the real moves you gotta make need to be done slowly and carefully. Otherwise you will just get 
Start by empowering unions and being a massive, vocal advocate for them. Show up to picket lines and be aggressive in your rhetoric. Put out ads and use your stump to say "your president wants you to start a union". Similarly, vocally support other protests.
Next, you move by carefully winding down programs and manipulating government agencies. End military vehicle sales to police departments. Tell ICE to sit on their asses and do nothing. Point all the three letter agencies at each other and suss out which ones are more capable and willing to work for you against the others. When you've got the FBI cracking down on CIA agents, you're going the right direction.
Meanwhile, go hard on enforcing laws that are on the books but go unused. Punish wage theft, union breaking. Enforce environmental regulations strictly to the point of being onerous. Crush white collar criminals. Prosecute the most openly corrupt politicians in both parties. This shit will make you extremely popular. The vast majority of Americans would absolutely love a strong man president sending politicians, bankers, and CEOs to jail.
All of this is going to accrue power and popularity in your direction. You'll shift the balance between the capitalist and working class - although the powers that be will obviously remain the powers that be. What you're doing is making yourself unassailable through popular support, improving what you can, and enabling the working class to build movements dangerous to the status quo that don't need you.
Then, in your second term, you go absolute sicko mode. In the event that your election is stolen through electoral bullshit, you don't do a Trump and go belly up. You refuse to concede. You take office and dare the state to stop you. If you're powerful enough, they'll be scared to.
All of this is, ultimately, driving towards your own assassination. That's ok - you are not building communism through the POTUS. You're making revolution possible. If you get got (which is totally possible), that could be the catalyst to get the revolution popping off.
If you show up day one and try to press the communism button then you'll just get removed from office and maybe sent to prison.
