By Kevin Murphy

KANSAS CITY, Mo (Reuters) - Calling the legislation harmful to the middle class, Missouri Governor Jay Nixon vetoed a bill on Thursday that would stop workers from being required to join unions or pay dues.

Legislative supporters of the bill said they would override Nixon’s veto, which was expected, but that would require a two-thirds majority vote. Final votes for the bill in the state House and Senate fell short of that margin last month.

The veto session is in September.

The bill would make Missouri the 26th state with a “right-to-work” law. Members of Republican-controlled legislatures in Missouri and other states tout the law as promoting business growth, but opponents say it undermines unions and hurts wages.

Nixon, a Democrat, appeared with labor union members near a Kansas City auto assembly plant on Thursday morning and planned a similar event with labor in St. Louis later in the day.

"This extreme measure would take our state backward, squeeze the middle-class, lower wages for Missouri families, and subject businesses to criminal and unlimited civil liability," Nixon said in remarks released by his press office. "Right-to-work is wrong for Missouri, it’s wrong for the middle-class – and it must never become the law of the Show-Me State.”

The Missouri House passed the bill by a 92-66 vote and Senate vote was 21-13, meaning an override would require 17 more votes in the House and two more in the Senate.

The sponsor of the bill in the House, Republican Eric Burlison, has said he will try to persuade lawmakers who voted against the bill to override the veto. Burlison said the bill would help Missouri companies compete for employees.

Proposed right-to-work laws have produced large and vocal labor union demonstrations in Missouri, Wisconsin and Illinois this year.

"Governor Nixon chose to use this issue as a political prop to grab media headlines instead of taking advantage of a strong economic development tool that is proven to add good-paying jobs,” Daniel Mehan, president of the Missouri Chamber of Commerce and Industry, said in a chamber website posting.

(Reporting by Kevin Murphy in Kansas City; Editing by Doina Chiacu)