In 1981 the professional air traffic controllers organization union thought reagan would support it in its strike because

The answer is because Reagan supported PACO'S effort in his 1980 campaign.

Explanation:

However, in 1981, Reagan considered the strike a peril to national safety and ordered the controllers back to work. The workers refused to come back to work and Donald Reagan fired 11.345 striking air traffic controllers who cleared ignored his order.

This was one of the most importants events in US labor history.