Democrats are trying to delay passage of the bill in the lower house by refusing to show up for sessions, The Indianapolis Star reported. The Republicans hold a 60-40 majority there, but House rules require a quorum of 67 to do business.
The Democrats and Indiana unions have support from elsewhere. The NFL Players Association accused Republicans of wanting to "ram through" the bill before the Super Bowl, scheduled for Feb. 5 in Indianapolis.
The bill would bar unions from collecting fees from non-members in their bargaining units.
Gov. Mitch Daniels says the law would encourage businesses to locate in Indiana, but union leaders say it would simply give workers who do not join unions a "free ride."
The Republicans hold a 37-13 majority in the Senate, and both sides expect the bill to pass eventually, the Star said.