Solarpunk

It's an unpopular position, but I think you should vote for the party you believe will do the best job.

The problem with the "a vote for X is really a vote for Y" mentality is that Z then has no motivation to support your interests. More plainly, voting Democrat in the US may well keep Trump out, but Biden will have no reason to support a progressive agenda: he has your vote by default and he knows it.

The Democrats and other "centrist" parties lean on this tactic heavily because it means they never have to worry about doing anything difficult, like getting money out of politics, nationalising health care, or standing up to fossil fuel companies. It's effectively how the Right captures the Left: just run a more extreme candidate on the Right.

The truth is that 3rd parties are incredibly powerful. Just look at UKIP here in the UK, or the NDP and Bloc Québéquois in Canada. They never win (nationally) but the major parties have to adopt their policies to keep voters from bleeding to these smaller parties. It's why Canada is more progressive than the US, why every national project there has special exceptions for Québéc, and why the UK left the EU: none of these parties needed to "win" elections to accomplish their goals. Fear of losing votes to them was enough.

The Democrats could be more progressive. They could have run an actual socialist in the last election and won, but they opted for someone who wouldn't really change anything. The only way they'll ever run a change maker is if they think you and others like you might choose someone else.