Some states are closed-shop states though, meaning that if a union already exists in a given company then to work there you are required to join the union. Others are right to work states meaning that you can choose whether to join a union or not. Consequently the unions are far stronger in close shop states than in right to work states. In many right to work states the only unions in operations at all are those in the public sector.
Also, decertifying a union is difficult. The laws as they are constituted right now are built to protect the union, not necessarily the workers. I have been in 2 decertification attempts, both failed. It requires that employees of their own accord circulate a petition to call for a decertification election. They must get 30% of the employee population to sign the petition. Then they notify the NLRB (National Labor Relations Board) office in their area. After the NLRB confirms the results of the petition they then call for a general vote. It can take a while (I have seen months go by in one case) to get to the general election phase. During this time the employees and the union may campaign for either side of the argument. Often people who signed the petition get cold feet and, under pressure from the union or other employees who still want to union, either vote pro-union or abstain from voting. In the vote all that is needed is a simple majority, but the majority required is NOT of the total employee population, rather just those who participate. If a facility has 500 workers and only 10 actually vote and 6 of them vote pro-union, then the union stays. Typically fewer than 1/3 of the employees in a given facility actually vote. The union makes sure that as many as possible of the employees who are pro-union vote. On the other side, it is basically a ground-swell movement and often many people either don't care, are afraid of some kind of retribution, or feel pressured to vote one way or the other so they just don't vote. The employer may not interfere on either side as it would constitute an unfair labor practice.
Not very long ago a bill was proposed that made it even harder to remove a union, requiring I believe that over half of the total population of employees vote to remove the union (in the above example, 251 must vote to remove the union), but that the employees could not be required to vote at all. So if not enough employees even voted then the vote was disqualified.
