Illinois Gov. Bruce Rauner (R) waited less than a month into his tenure before delivering the first blow to public sector employee unions, issuing an executive order Monday that will prevent those unions from collecting fees from thousands of state workers.

The order will block unions from collecting money from public employees who are not union members but who pay fees to be represented. State law allows public employees to decline to join a union, but those who do must pay so-called fair share fees to cover the cost of representation; more than 6,000 nonunion state employees currently pay those fees.

“Forced union dues are a critical cog in the corrupt bargain that is crushing taxpayers. Government union bargaining and government union political activity are inexorably linked,” Rauner said in a statement Monday. “An employee who is forced to pay unfair share dues is being forced to fund political activity with which they disagree.”

Rauner’s order does not apply to state employees who choose to be union members. The American Federation of State, County and Municipal Employees represents about 42,000 state employees who still pay dues.

Labor officials said Monday they would challenge Rauner’s executive order in court. Roberta Lynch, executive director of AFSCME’s Illinois branch, called the order “a blatantly illegal abuse of power.”

“It is crystal clear by this action that the governor’s supposed concern for balancing the state budget is a paper-thin excuse that can’t hide his real agenda: Silencing working people and their unions who stand up for the middle class,” Lynch said.

But Rauner got to court first: His office on Monday filed a preemptive lawsuit against dozens of unions with state contracts seeking approval for the order.

The executive order presages what is likely to be a bruising battle over AFSCME’s next contract. The current contract runs out on July 1, and Rauner’s order serves as an early warning that he will take a tough line with the powerful union.

Rauner made no effort to hide his disdain for public employee unions during his 2014 campaign, in which he ousted Democrat Pat Quinn. Rauner said the contracts agreed to between unions and state officials are major contributors to the huge budget and pension liabilities Illinois deals with.

Rauner cited those unions and their political power in Springfield in his State of the State address last week. “Government unions should not be allowed to influence the public officials they are lobbying and sitting across the bargaining table from through campaign donations and expenditures,” he told legislators during the address, when he proposed right-to-work legislation that would allow state employees to choose not to join a union.

The same unions spent millions of dollars on Quinn’s behalf against Rauner during the 2014 campaign. The Illinois Education Association spent more than $4.5 million against Rauner, and other unions contributed another $3.4 million to an outside group that attacked the Republican.

Rauner is the latest Midwestern Republican to target public employee unions. In recent years, Indiana’s then-Gov. Mitch Daniels (R), Wisconsin Gov. Scott Walker (R), Ohio Gov. John Kasich (R) and Michigan Gov. Rick Snyder (R) have all moved to curtail unions’ political power, either through legislative action or executive order.

Rauner is the only one of the five, however, to be constrained by a Democratic legislature. The legislature could vote to overturn Rauner’s order.