Correction: An earlier version of this post included incorrect information about when the measure would be on the ballot. If it passes the legislature, the vote would be in August.

The national battle between conservative groups and big labor unions is moving to Missouri as outside groups on both sides gear up for a vote that could come as early as Monday on controversial legislation that would strip unions of the power to compel workers in union shops to join.

The legislation, known as Right to Work, would prohibit requiring payment of union dues as a condition of employment. Unions in the 24 states that have passed Right to Work have seen sharp drop-offs in dues-paying members after the law takes effect.

The Missouri bill would put a Right to Work proposal on the August ballot. Voters defeated a similar Right to Work measure in 1978.

But despite the fact that Republicans control a super-majority in both chambers of the Missouri legislature, the proposal did not win enough support in a test vote earlier this week to advance to the Senate. More members voted for the bill than against it, by a 78-68 margin, but Republicans didn’t meet the 82-vote threshold that represents a constitutional majority.

Missouri legislative rules require a constitutional majority to pass any bill. That means state Republicans are scrambling to find the four remaining votes necessary to pass the bill before the legislature adjourns at the end of May.

To find those votes, Republicans will look to their own ranks: Nineteen Republicans joined minority Democrats in opposing the legislation. Two Republicans voted present, avoiding taking a position. And nine Republicans simply didn’t vote.

“There’s a substantial and notable bloc of Republican House members who are dead set against it,” said Jeff Mazur, a spokesman for the American Federation of State, County and Municipal Employees, which is lobbying against the bill.

State House Speaker Tim Jones (R) has told allies he could bring the measure up again as early as Monday, if he can secure the four extra votes he needs to win passage.

“Some people are concerned about the perceived demographics of their districts. Some people feel the labor influence is stronger in some districts than others,” Jones said in an interview. “You have to set aside the politics, and you have to decide what is good economic policy for the state of Missouri. I’m trying to keep them focused on jobs and the economy.”

Jones is getting help from outside conservative groups who have made Right to Work legislation a top priority. The Missouri chapter of Americans for Prosperity rallied for the bill at the statehouse in Jefferson City in January. Grover Norquist, head of Americans for Tax Reform, sent a letter to members of the legislature, while FreedomWorks asked members to call nine on-the-fence Republicans, only two of whom eventually supported the bill. The American Conservative Union said it would “score” the vote and hold Republicans who voted against it accountable.

“It’s a safe vote,” Norquist said in an interview. “When people realize you’re more likely to lose a Republican primary than a general election on a vote like this, the number of Republicans willing to vote for Right to Work will get closer to 90 percent.”

But some members say the outside pressure is having the reverse effect. Rep. Ron Hicks, one of the Republicans who voted against the Right to Work legislation, said 70 percent of the calls to his office urged him to vote against the measure.

“The bullying is just not going to stop,” Hicks added. “This is ridiculous. That’s what’s wrong with this country: Too many influences, too much money.”

“I understand Right to Work is a big issue across the nation, but I think I should still be able to vote the way my constituents want me to vote,” he added. “This one time I’m going outside the box to stick up for my constituents and all the sudden I’m a RINO? I’m a Democrat? I’m a zombie in Democrat clothing?”

Even if the bill passes the House, there’s no guarantee it will get through the Senate. Democrats have promised to filibuster, though Republicans have sufficient numbers to cut off debate — if the caucus doesn’t fracture like the House has. Because the bill would put the matter before voters, Gov. Jay Nixon (D) would not have the opportunity to use his veto pen.