Less than half of the US population voted in the last election. ~50% of Americans _who voted_ voted for the party that is diametrically opposed to unions, but that group is only ~23% of _Americans_.
This also matches what Democratic-leaning pollsters like David Shor have said in their own analyses.
1. Get people to change sides. This is difficult: people who already vote in a particular way are unlikely to change because of external pressures (and in fact, are more likely to dig their heels in if challenged on their views); or
2. Get more people to vote. This can be challenging in its own way, but can also just boil down to logistics problems.
Whether or not 100% voter turnout would have the same results is neither here nor there. If you accept that premise + the premise that the vast majority of American voters vote down the party line for one of the major two, then you have potentially 2x the number of voters for your desired outcome. All you have to do is go find them and get them to vote.