I found that acknowledging "yeah, we are the tax and spend party, as opposed to the opposition that is don't tax, spend anyway" works really well. GOP legislation invariably relies on fantasy or outright bad math to explain how it will somehow pay off. The ACA was a thousand pages...a hundred pages of what it was going to do and 900 pages of where the money was coming from. That's responsible.
That is literally how Hillary conducted her campaign. Every platform position she held was supported by economic analyses by experts demonstrating projected benefits to the economy and how those benefits would either reduce the deficit or at least end up being cost-neutral. She was, in every sense imaginable, the quintessential, consummate technocrat: vote for me because I am the most educated, most qualified, and most demonstrably competent candidate for the job and here are my 2500 binders of economic analyses compiled by my army of experts to demonstrate that. She lost. Horribly. To the guy who spent the entire election cycle making vague proclamations about how he's going to build a giant wall (whose precise cost even he couldn't keep consistent in his head) and that he was going to get the Mexicans to pay for it...somehow, and when he was asked to specify that "somehow," he made vague overtures about trade deficits and tariffs (again without anything in the way of specifics). And despite it being demonstrated, time and time again, that he has no idea what he's doing, that he acts impulsively and without stopping even for a moment to consider the costs or consequences of his decisions, and despite the fact that this lack of foresight continues time and again to blow up horribly in his face to the detriment of the millions of Americans he is duty-bound to serve, he still maintains a consistent base of enthusiastic voters, who remain convinced that he is a mastermind and a shrewd businessman, and that his policies, despite all evidence to the contrary, are working flawlessly.
I think it's time to recognize that most people don't care about whether a policy is "economically sound" or has been "vetted by policy wonks and think tanks." They care about if it sounds good (in an inspiring/energizing way) and if it aligns with their personal morals and groks with their worldview of how things
should work. This isn't to say that politicians should disregard whether or not their policy goals are viable, or should adopt a Trumpian policy-by-twitter-fiat approach to governing. But rather that if the Democrats want to win elections they need to get away from this technocratic "we had experts run three million monte carlo simulations and forecasts demonstrate that this policy proposal is consistent with a 4% increase in GDP growth year-over-year versus the policy our opponent is proposing," and onto selling things in a way that are exciting and motivate people to go to the polls and vote.
By the way, the above is also why the Republicans continue to hold popular support, despite being obviously hypocritical as regards being "the party of fiscal responsibility," and also despite championing policies that demonstrably harm their electoral base. Electoral politics aren't about who
did what, but rather about what you're telling prospective voters and how you're telling them. It's a big part of why I think discussions about, say, healthcare, really need to get away from fiscal discussions, and into:
a) moral arguments (nobody should have to choose between utter economic ruin and not dying; if we purport to be the greatest nation on Earth then we should start treating our citizens as such; etc.)
and
b) Elaborations on day-to-day quality of life benefits a voter will enjoy (i.e.: everything is covered and you never again have to worry about how you're going to pay for it; imagine getting into an accident and not having to worry if the ambulance that will pick you up is covered by your insurance, or dreading surviving the emergency and having to spend the next 6 months fighting with your insurance company to get them to do their job). I think this is also, incidentally, a big part of why I hate the Medicare access for all crowd. In addition to it being a horsehockey half-measure, it also cedes the political high ground. Nobody actually cares about their "plan," and nobody actually wants to keep their plan; what they care about, rather, is coverage and how their ability to get consistent, reliable and affordable healthcare will be impacted. Medicare for All allays concerns about this. Medicare Access for All doesn't.