JEFFERSON CITY (AP) � Missouri Gov. Jay Nixon on Friday vetoed legislation requiring public employees to annually reauthorize paying their union dues through paycheck withholdings, setting up a confrontation with Republican leaders who said they have enough votes to overrule him.

This is the third time the Democratic governor has vetoed legislation that would change the rules for unions, which represent about 257,000 Missouri workers � nearly 10 percent of the state�s workforce, according to the U.S. Bureau of Labor Statistics. The figure is for all workers represented by a union and does not give a breakdown for private companies and public entities. Nixon has vetoed 138 bills since he took office in 2009, and 34 became law after the legislature overrode the vetoes.

Nixon in 2013 vetoed a similar bill, which lawmakers failed to override. And last year, despite GOP supermajorities in both chambers, lawmakers failed to overturn his veto of so-called �right-to-work� legislation that would bar private-sector unions from requiring workers to pay fees.

The Democratic governor said this year�s bill piles unnecessary burdens onto public unions. He pointed to a requirement for unions to retain internal information as searchable, electronic records and allow employees to sue if they cannot access them.

Republican supporters describe the measure as �paycheck protection,� arguing it would make unions more responsive to member needs and opinions.

Nixon called such concerns a pretext to interfere with unions, noting that public workers can end the paycheck withholdings at any time.

�The animus towards� unions �underlying this legislation is clear,� he wrote in his veto message.

Nixon also condemned the bill for �inexplicably� exempting first-responders from the requirement to reauthorize paycheck withholdings.

�The legislation picks and chooses who to �protect� based on political calculations rather than a rational basis,� Nixon wrote.

Rep. Holly Rehder, the Sikeston Republican who sponsored the bill, said she would have preferred to include first-responders but that it was a compromise �that was necessary again this year to make sure we had ... everyone involved in approving the legislation.�

The Missouri Chamber of Commerce President Dan Mehan said the current system of withholding dues from workers� paychecks is a �racket� that benefits politicians the unions support.

Lawmakers would need a two-thirds vote in each chamber to override Nixon�s veto. The Republican-led House and Senate both just met that threshold when passing the bill earlier this month. Rep. Courtney Allen Curtis and Sen. Maria Chappelle-Nadal, two St. Louis Democrats who voted for the bill, both explained their support by saying unions discriminate against black workers.

Rehder expects the override to succeed. �I think it�s a pretty solid group,� she said.

Lawmakers are on spring break, and they are to return to work March 29. Their session runs through mid-May.