That sounds abusive.

In the report linked above there is a section on Plant-Closing Threats. They point out the following:

Employers recognize the value that plant-closing threats add to an overall anti-union strategy, and they frequently issue such threats even when they do not intend to carry them out. Bronfenbrenner (1997) found that half of all employers involved in union organizing campaigns nationally issued threats to close all or part of their operations should the union be victorious. Just three percent actually followed through on these threats after workers voted in favor of union representation. (Bronfenbrenner 2000)

There is also a section that discusses the legality of such anti-union strategy:

Most Employer Tactics Are Considered Legal or Difficult to Prove as “Unfair”

At first glance, many of the tactics that employers use appear to violate the law. For example, the threat of company closings or relocations is one of the most common anti-union tactics used during organizing campaigns. The following are examples from the CRC Survey of the ways in which employers delivered the threat that avoid explicitly connecting the closure of the facility to employees’ decision to join or vote for the union or engage in protected activity:

• “I can close this company if I want to.”

• "I might have to close [the company] because we don’t have money. I might shut down [the company] and re-open it under a different name.”

• "The company is losing money and can’t compete with a union in place.”

• "The company next door closed because of a union. A lot of companies are closing down or leaving the U.S. because of the unions.”

• “Unions make our business unviable”