A more thorough explanation:

The Midwest Piping Doctrine is a policy followed by the National Labor Relations Board (NLRB) that requires employers to remain neutral in situations where there are rival unions competing for representation of employees. This policy was established in the case of Midwest Piping & Supply Co. in 1945 and was followed for 37 years until 1982.

For example, if two unions are competing for representation of employees at a company, the employer cannot show favoritism towards one union over the other. The employer must remain neutral and allow the employees to decide which union they want to represent them through a fair election process.

In 1982, the NLRB modified the Midwest Piping Doctrine in the case of RCA Del Caribe, Inc. The new policy requires employers to recognize and bargain with an incumbent union, even if a rival union has filed a valid election petition. However, if there is no incumbent union, the employer can remain neutral until a valid election petition is filed.

For example, if a union is already representing the employees at a company, the employer cannot refuse to recognize and bargain with that union just because another union has filed a petition for an election. The employer must continue to work with the incumbent union until a new election is held and a new union is chosen by the employees.

Overall, the Midwest Piping Doctrine and its modifications aim to ensure fair and democratic representation of employees by unions, without interference or bias from employers.