Gov. Jay Nixon has vetoed a so-called paycheck protection bill, setting up a possible veto override effort before the legislative session ends in May.
The bill, House Bill 1891, would require public employees who are union members to opt in each year before union dues are deducted from their paychecks.
Backers of the bill say it makes unions more accountable to their members. Opponents say it threatens the viability of unions.
Nixon, a Democrat, said in a veto memo Friday that the bill would create challenges to employees trying to bargain collectively.