Chiefly, a major change in U.S labor law.
About 48% of workers in the U.S say they want a union, whereas only 10.7% of workers in the U.S are in unions. If everyone who wanted to be in a union could join a union, the union movement would be in great health.
The problem is that it’s incredibly difficult to win a union election if employers decide to fight unionization, because current labor law allows employers to conduct coordinated campaigns of intimidation with minimal penalties (indeed, penalties are so low that employers just budget in the fines as the cost of doing business).
In terms of fixing this, the proposed Workplace Democracy Act would help. It’s not a silver bullet – there are other issues, such as the need to update penalties for labor law violations, the understaffing of labor law enforcement, etc. – but it covers a lot of the big ticket issues.
Then again, there are those who think the U.S labor movement needs to think bigger than just fixing elections systems.