America isn't poorer than it was before, the funding could be had.. but I don't think most folks would want to make any sacrifices in order to free up money to fund truly large space programs.
As I mentioned, people making those sacrifices would do the opposite of what you're trying to achieve. If they did, that would mean that consumer spending goes down, and if consumer spending goes down, less people are making money, and if less people are making money, the government has reduced tax revenue. Remember, what people produce in America counts as GDP, and people don't produce if nobody buys.
Having said that, while it's true that the government today is taking in slightly less as a percentage of GDP, you've got to consider that the GDP is much greater now than it was in the late 60's, even after you adjust for inflation. In fact if you just adjust government revenue for inflation, then in terms of real dollars the government has twice the revenue today than it had in 1968, as you can see here:
And also if you look here, the government makes by far the most amount of its money from personal income taxes, which are heavily influenced by consumer spending:
And you know the biggest reason why we have a bigger GDP (and thus more government revenue) than we did back then? It's because we have better technology that makes us better producers. And before somebody argues anything about our population growing, I've got you covered there too:
Notice the GDP per capita (that is, GDP per person) has only risen since the 60's, and boy has it risen by a LOT. And again, that's due to individual people having better technology, which includes little things such as smartphones and other creature comforts that the OP was lamenting.