Liberals will lie to you and tell you to "just vote in the primary," but in reality they simply don't care about trans rights and do not believe there is ever a right time to challenge transphobic Democrats.
First primary: the transphobes hide their colors so you can't vote against them.
First general election: the snakes reveal their colors, but you can't vote against the traitors because "blue no matter who." They tell you to just vote in the next primary.
Second primary: the same people who tell you now "save it for the primary" shift their tune to "we can't give up an incumbent seat in a purple district!"
Trans people can see through these bad-faith lies. Yet of course, mansplaining trolls will be sure to breathlessly explain "lesser of two evil" math, as if that's not an argument we haven't heard a thousand times and already rejected due to its obvious flaws. There's never a shortage of arrogant cis people who think they know more about trans issues than actual trans people.
Liberals want your votes, not your voices. Be a good little minority and sit down. Shut up. Support the party. You owe the party your support. The party owes you nothing.
EDIT: A few thoughts to address the comment in the thread here:
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You need to get past your team sports approach to politics. I am not advocating voting against Democrats in general. I'm advocating for not voting for the 1% of Democratic candidates that have chosen to become fascist collaborators and adopt Republican positions on trans rights. To trans people, the Democratic party going TERF is a greater danger than the Republican party winning a slim majority in the Senate. The nightmare scenario here is what happened with Labor. Labor has completely fallen to Nazi collaborators. The UK has regressed in trans rights to now be the most transphobic country in Western Europe, and that happened under the rule of a center-left government. The trans people of the UK would have been better off with another term of conservatives than the transphobic labor leaders they elected instead. We don't need to elect more Fettermans who will turn around and vote against the party the first chance they get. So many people in this thread believe, "you have to vote for the Nazi collaborators, otherwise the Nazis will win!"
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On saving democracy: you are vastly underestimating how difficult that task is. You seem to imagine you can just write a bill that just says, "democracy saved," and call it a day. Actually saving democracy is going to require stripping powers from the presidency and dismantling the post-9/11 security state. That requires Democratic leaders of moral strength and conviction. It will take leaders willing to look their constituents in the eye and tell them, "I am going to vote for this bill to dismantle ICE. Yes, that means that occasionally an illegal immigrant is going to murder someone. I'm going to vote for this bill to dismantle the NSA's illegal programs. Yes, that means we may have more terrorist attacks than we would otherwise. It's worth it to save our Republic." Saving democracy requires leaders who are literally willing to vote to kill their fellow Americans. Every power you strip away results in a few more people dying. It's inevitable. And these transphobe Democrats are fundamentally cowards with no inner moral center. They'll follow any poll or vote for any policy they think will get them power. This kind of person is not capable of saving democracy. You can't defeat Nazism by voting in Nazi collaborators, and it's insane to pretend you can.
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Those arguing that trans rights are a minor or insignificant issue? Frankly, if you believe this, you deserve to have your right to vote taken away. If you're not willing to protect the vulnerable among your community, you deserve whatever evil befalls you. Karma's a bitch in that way. We can all go to Hell together if that's the path you choose.

I love this question, because the answer can be so many things depending on your set, setting, and availability. Engage in local politics and community support.
Show up for meetings and listen. Share your thoughts and opinions as you become informed or find you have questions. Talk to your friends and neighbors. Join a local committee. Run for a local office or go further if that's your path.
Get acquainted with your local town office, rec center, senior center, etc. Start volunteering. Serve meals, maybe deliver some. Support a food bank - donations, pickup from local business donors. Promote Progressive voices that are supporting workers' and people's rights. Support people doing everything described in the previous paragraph.
Gogol Bordello said it first: Think global, act local. Installing a president with true progressive values isn't a path to success unless there is wider congressional support, so start developing grass roots support or support those that are developing such.
Conservatives have spent 50+ years slowly and steadily doing this and fucking over progressive voices all along the way with exuberant neoliberal support on embracing corporate money and influence. It's a long road back. We need to stop accelerating towards stupidity and it's the right wing authoritarian cultists that have their foot stuck on the gas pedal so keeping them out of power would be super neato as far as I'm concerned.
Connect the dots, please. Those are all wonderful things, but how do they add up to avert the authoritarian takeover of national power that the conservative movement has been working on for so long?
Local power matters. Both in local politics, like city and county, and in community connectedness. Make your town a better place, create non-political systems for neighbors to help each other, gain control of the narrative. You're right that conservatives have been working for so long, and nothing we can do will magically flip a switch. The solution is going to take time too. And that starts with building connections with your neighbors.
I live in a city with the last vestiges of a progressive 3rd party started in the 1990's in the upper Midwest to do just that, but the two major parties collaborated to crush it. But we do those things locally. Lots of people do those things. It just doesn't translate to state and national politics.
Case and point, here I am encouraging people to organize and make changes outside of the electoral system, and getting told in not so many words to pound sand.
I don't believe you. I believe that they tell you to pound sand if you say to organize instead of voting, I don't think anyone is opposed to organizing and voting.
Exactly. Plus, imagine a progressive president just suddenly takes over today. Senate and House are both the same. All of the money interests would love nothing better than to work together across the aisle and make sure the progressive president is the least effective president ever to a) preserve their money and power l, and b) propagandize the everloving fuck out of telling the voting population that progressives are useless leaders, hampering the movement for another generation or more. Upsetting the current power structure in the US needs support starting at the grass roots level and building upwards.