FWIW, I know you're to the left of me on some issues, and on others I assume we'd disagree on prioritization. But neither of those things are relevant to the argument I'm making here.

I strongly disagree that backing third party candidates in presidential contests in close, contested states is an effective way to advance progressive causes - in the short, medium, OR long term. I'll divide my argument into a few main tacks:

1) Impact

The value, to progressive values, of electing moderate liberals over conservatives, even in today's absurdly polarized environment, bears repeating. In particular, note two issues that the Supreme Court has tackled in the last decade and will continue to address: the financing of campaigns, and access to voting rights. These are foundational issues in our Democracy, because they speak to how elections happen, and in turn they help determine how we are governed. As Bernie Sanders noted repeatedly during his campaign, if you care about any other issue at all, you need to care about these issues first.

Say what you will about literally any other policy points, but getting better Supreme Court rulings on these matters will make any progressive change easier to implement. Conversely, electing Trump probably means that the current rulings are in place for a generation. Those are pretty high stakes.

2) Incrementalism

Setting aside those critical issues, take a look at some of the other achievements of the Obama administration, which will likely to be continued by a Clinton administration. On many issues, such as healthcare or the environment or financial regulation or tax policy, Obama is seen as a disappointment by many on the left. But the progressive shifts he's effected on these fronts should rightly be seen as the sort of incrementalism that defines change in America.

Pretty much every progressive change in American history has been incremental. Health care is probably the most obvious example: we went from nobody having guaranteed access to health care, to just military veterans, to military veterans and seniors, to military veterans, seniors, and the extreme poor, to military veterans, seniors, the extreme poor, and children, to finally everyone (albeit in a patchwork system that still needs work). It took about 80 years. And the work continues, but that's how things happen.

I don't ask you to be happy with incrementalism - I am happy that you are not happy with it actually, as we should always have people advocating for faster and bigger change to move the window of the acceptable forward - but we shouldn't be surprised by it. More importantly, we shouldn't reject it as meaningless. It's not.

3) Privilege

This isn't an argument, per se, but an observation (and one that I imagine you've seen before). If Trump wins, you and I will probably be OK. This is because we're both relatively well-off white dudes who aren't in any heavily oppressed group. But if you really care about progressive values, you should care about those who will NOT be OK in a Trump presidency. People whose residency or marriage or dignity will be in jeopardy can't afford to wait out 4 or more years in the interest of some long goal. We have some responsibility to vote with their interests in mind.

4) Backlash

If Hillary loses in 2016 because some progressives support Jill Stein or somesuch, the reaction from mainline Democrats won't be "oh, gosh, we should really listen to the left more next time around". No, the reaction will be anger and resentment followed by turning their backs on the left. Conversely, the reaction from the progressive left will not be to proudly tout the electoral power they wield. It will be to deflect blame for the disaster they have a hand in, and saying that the loss was for other reasons.

How do I know this? That should be easy and obvious - we saw all of this happen 16 years ago. Almost 100,000 Floridians voted for Nader. If 1% of them had been tactical in their voting (and please don't give me that crap about how a majority of them didn't prefer Gore over Bush) then the history of this century is a lot different. And the reaction wasn't a leftward shift in the Democratic Party. It was a profound backlash against the left that didn't really resolve itself until anger against the Bush administration swamped everything else in the 2006-2008 cycles.

The right way to effect change for progressives isn't to mount third-party general election presidential challenges. This should be obvious not just because of Nader. It should be obvious because of Bernie Sanders! While I may have some minor quibbles with how Sanders has comported himself, he's provided a nearly perfect blueprint for how to push Democrats to the left without screwing them in the general election. He switched parties to run as a Democrat in the primary, ran a issue-heavy campaign and forced the other candidate to move left in substantial ways, and then at the end of his candidacy he used the leverage he had in delegates to enshrine many of those principles in the platform.

By any standard short of "electing Bernie Sanders president", Sanders' candidacy was a huge success for progressive values. In this election, of all presidential elections, we should be able to see with clear eyes what the best way to advance progressive values is.