In my opinion by limiting your question to "defending liberal democracy" you are also vastly limiting potential remedies. The current resurgence of authoritarianism takes advantages of weaknesses of liberal democracy. "Defense" of democracy necessitates making the system smarter and more resilient to these authoritarian takeovers. 

"Defense" may necessitate a transformation of the status quo. 

Exactly what makes liberal democracy weak? One reason is poor collective decision making. Examples of bad collective decision making include:

1. Democratically controlled states inability to grow. Anti-growth and high cost-of-living policies of California, New York, and other blue states have encouraged millions of their residents to move to Red states. This foot-voting is a clear indicator of the incompetence of Blue-state policies, and simultaneously hands authoritarians like Trump more power.
2. Obviously the election of authoritarian Donald Trump who hopes to end democracy as we know it.
3. Incompetent internal party decision making systems that elected Joe Biden to be the presidential candidate in 2020, whose utter unpopularity, inability to engage in modern-style social-media driven policies, and senility was a large contributing factor to Democratic Party losses in 2024. 

Whereas EA has funded some pro-approval voting initiatives in the past, it has not:

• Engaged in any small scall randomly controlled trials to validate claims made.
• Evaluate other voting reform alternatives such as Condorcet methods, multi-member districts with Single Transferable Vote, evaluate the possibility of proportionately representative systems, etc
• Evaluate the potential for "Deliberative Democracy"
• Evaluate the potential of Citizens' Assemblies as used throughout the world in Ireland, France, UK, Canada, etc.
• Evaluate the possibility of more direct democratic alternatives such as sortition where decisions are chosen by lottery. 

As far as bothering to test and validate potential reforms:

• Lobby labor unions to implement improved decision-making reforms.
• Lobby political parties and organizations to implement improved decision-making reforms.
• Lobby HOA's to implement improved decision-making reforms.
• Fund research into modeling and simulation of improved decision making with respect to mechanism design, economics, and social choice theory. 

Lobbying smaller organizations may be more effective than attempting to run a full city or state referendum campaign for change, and build evidence whether interventions are actually effective or not.  

In my opinion pure voting system reforms (such as approval and ranked  choice voting) have low probability of success because they don't tackle the core decision making problems of liberal democracy. Voting system reforms might improve aggregation, but if a majority of voters simply have incorrect information, they will still arrive at incorrect decisions. The only reform I've found that tackles the problem of voter ignorance is sortition, which I've linked above. In short, you can improve the decision making capacity of voters by paying them, and giving them enormous resources to arrive at better decisions. And the only way to scale such a process is by reducing the number of participants through a fair democratic lottery. As far as organizations advocating for this, it includes: Ofbyfor, Democracy Without Elections, INSA, Assemble America, BANR, etc. Full disclosure, because I believe sortition has the greatest likelihood of success, I volunteer in some of these organizations. 

Ultimately we need a system that its own citizens believe in. People like Donald Trump succeed because citizens believe that the status quo is so bad, Trump is a valid alternative. Authoritarians succeed when liberal democracy fails its citizens. A successful defense of democracy demands improvement of democracy. 

The think tank Power For Democracies recently published its first donation recommendations for safeguarding democracy, including a U.S.-based one, Freedom2Vote. Their website also provides details on the evaluation.

I believe Power for Democracies is doing EA-style evaluation in this area (including the US?) building off Effektiv Spenden's work in Germany

Digital rights organizations like the Electronic Frontier Foundation might be of particular interest, as they not just combat anti-democratic abuses by both state and corporate powers, but are particularly interested in protecting spaces of communication from surveillance and censorship, which seem of particular importance for making society resilient to authoritarianism in the long term, including in the least convenient possible world where democratic backsliding throughout the West turns out to be in fact a durable trend (in which case the more traditional organizations you cite will probably be useless).

An important point: this is not just a US problem, but a global struggle to defend democracy worldwide wide. There has been a massive underinvestment over years  in democracy infrastructure, democracy defenders, and civil society - and we are now reaping the fruits of that world wide. The far/alt-right and Autocracy Inc. understand this extremely well. There has to be a push to defend democracy world wide.

For more, feel free to reach out at [email protected]

What about funding a project trying to end tax-deductible donors-advised funds? Things like DonorsTrust are what brought us here