First past the post can only be beaten if you manage to swell enough support for a third party to be viable and not split your vote. And because of maths, this is a huge obstacle to overcome.
Some back seat democracy for ya (I am not from the US, from Australia): you need to hammer on about voting reform, and mostly voting reform, until you have it. This needs to be the number one issue in everyone's minds until you have it.
"We need to beat Trump" is a band-aid at best, and is unlikely to win over any conservative voters to your cause (and, you will need them to get a third party to have any chance, in my opinion).
Your system is beyond repair without it (in my opinion).
Y'all need preferential voting, and/or proportional representation. Or whatever change means future reform is possible.
You already have multiple parties, who each stand no chance against the spoiler effect without a massive campaign to get democratic voters (and maybe some Republican voters) to switch to it.
Anything short of that means you'll get another Republican government.
And at this rate, this means your barely-counts-as-a-democracy will be dead.
Just here to point out that the 3rd party candidate with the most votes was Jill Stein with a whopping half of a single percent of the total votes cast.
There is no viable 3rd party. And there won't be until they start working to get elected at the local/state level and start getting members in Congress.
This is so often overlooked. A 3rd party could win EVERY SINGLE AVAILABLE SEAT in a presidential election year and they would still only control the executive - Congress and the judiciary would still have a supermajority of not-thems, and you couldn't change that in one term, or even two. Not to mention every governor, state legislature, county board...