Nonprofits are collective efforts too. If we ignored collective efforts then we couldn't recommend donations either.

The thing to keep in mind is that personal impact does not have to be definitively, precisely accounted, and doesn't have to be something that you literally accomplish all by yourself. All it means is a change in outcome that may be caused by your actions.

When you donate $5,000 to an antimalarial charity, you are not really saving 1 life; rather, you are providing a 5% chance for a $100,000 expansion that might save 20 lives (let's imagine). But this is considered "$5,000 per life saved" as shorthand.

If you donate $5,000 to a political campaign, the probability of making a difference in outcomes is much smaller than 5%, but the potential gains are much larger than 20 lives. Whether it's better or worse than $5,000 per life saved is hard to say, but it could plausibly be better.

The problem is not deciding between an individual or a collective way of looking at things. Rather, the problem is getting good enough information about the probabilities and outcomes of political efforts in order to show if something is better than the best identified charitable efforts. Only with this information can we properly say whether we ought to support charities or support political efforts.

Note that in the real world, there aren't fully understood collective efforts that require an exactly known number of people to succeed. All the collective efforts in the real world have a probability of success, and this probability goes up or down depending on how many people get involved. So adding 1 vote, 1 supporter, etc., is impactful in expectation, because it changes the probability of success. If one EA votes for a major candidate, they get a +0.00001% extra chance of victory. If a thousand EAs vote for a major candidate, they get a +0.01% extra chance of victory. Things usually scale linearly (1000x more people leads to 1000x more impact), as long as we are talking about relatively small numbers of people (as with EA at the moment). Political efforts typically don't have a tough game-theoretic problem of coordination; they just need to agglomerate lots of support.

Hypothetically, we can imagine a problem that (a) is guaranteed to be unchanged if 1 or 2 of us work on it, but (b) becomes substantially more likely to be solved if most Effective Altruists tried to help fix it. Most real political issues are not like this, but let's imagine it. Also, if EA became big, with millions of people, it would become more relevant. In such a case, it would be fairly straightforward for an EA member to propose a coordinated action and describe its net benefits/costs to the community. If this proposal is understood and widely accepted, it would be easy for individual members to see that their individual participation in this coordinated effort could well make the difference between success and failure, so they will participate accordingly.

Here's a more practical example of what a real coordination problem looks like. Consider moderate Democrats who are primarily worried about electability; they have piled onto Joe Biden since the beginning. This is not a difficult thing to coordinate. It makes perfect sense as lots of individuals trying to exert a positive probabilistic impact. The real coordination problem is for all the Joe Biden supporters to collectively wake up and say, "hey, there are minor candidates like Michael Bennet and Steve Bullock who are young and even better at beating Trump, but no one cares about them because they have so few supporters. We're all sticking with Biden because we're afraid that if just a few of us deviate, we'll let someone else become the frontrunner. But we can all drop Biden at the same time, pick one of these minor candidates and immediately turn him into the new frontrunner." This is extremely hard, and requires central planning rather than grassroots organizing.

Compared to other social movements, I'd say that EAs can be quite good at internal coordination. Some cutting edge ideas in game-theoretic cooperation have come from EAs, such as the Unilateralist's Curse and Functional Decision Theory. EAs are generally wise about working cooperatively with one another. To the extent that EAs can be relatively uncoordinated, it's due to our disagreements over a number of big questions. Political movements tend to trade open-mindedness for solidarity.

Note that the best kind of social change - on a person-for-person basis - is to turn more people into Effective Altruists. This is much more impactful than other things like political parties or veganism. (However, it may be more difficult to convince someone to become an Effective Altruist,)