A fight has broken out between the nation's most prominent Muslim advocacy group and the labor union attempting to organize its Washington staff.

Service Employees International Union Local 500 said in filings Wednesday that the Council on American-Islamic Relations was trying to bust its effort to organize the civil rights group's staff. CAIR responded with a statement Thursday calling the charge "meritless." The conflict comes just days before a scheduled April 24 workplace organizing vote by CAIR's staff.

Maryland-based SEIU Local 500 filed the unfair practices charges against the Muslim advocacy group with the National Labor Relations Board, the main federal labor law enforcement agency. The union said, among other charges, that it was denied access to the workplace and said CAIR's officers made coercive statements to workers regarding the election and discharged at least one for organizing. The filings were posted on the labor board's website, though the actual complaints are not public.

"Disappointed that a civil rights group like #CAIR is trying to stop its employees from even voting on having a union," SEIU Local 500 said on its official Twitter account. "Workplace rights are #civilrights - which makes @CAIRNational #hypocrites for policy of stopping employees from being able to vote for union," it said in a follow-up tweet.

CAIR, which is well-known for its aggressive efforts to target anti-Muslim bias, responded that it "strongly supports the labor movement" and expects the charges to be dismissed.

A spokesman for the union could not be reached for comment.

The union has been trying to organize the advocacy group's workers since October, labor board filings show. CAIR objected to the effort, arguing that it was a religious organization and therefore exempt from the National Labor Relations Act. A regional administrator for the board rejected that argument this month, stating that CAIR was "more akin to a secular civil rights group" and its workers were therefore free to unionize.

In its statement Thursday, CAIR indicated it was still rejecting the board ruling, saying that it was acting to protect its "legal identity as an American Muslim civil rights organization."

The union's efforts put CAIR in an awkward position, as it has generally allied itself with other liberal groups. Opposing the workplace election opens it to charges of anti-union bias. Not opposing it means accepting a legal precedent that it is not covered by some religious exemption laws.

SEIU Local 500 had hoped that the earlier labor board ruling would end any conflict between it and the union. "This is a great opportunity for CAIR... Like other similar groups, CAIR's staffers should have an opportunity to have representation," spokesman Christopher Honey told the Washington Examiner this month.

The April 24 election covers only the workers at CAIR's Washington headquarters. Honey put the number of workers in the bargaining unit at less than 50.

CAIR's statement to the Examiner reads:

"The Council on American-Islamic Relations (CAIR) strongly supports the organized labor movement, which has played an indispensable role in creating a more just and fair work environment across our nation.
"In an effort to avoid the election, the union filed a meritless charge. A substantially similar claim was dismissed a few weeks ago. This new meritless charge is expected to be similarly dismissed.
"CAIR has remained neutral throughout this process and respects the free choice of employees. We look forward to the election going forward as soon as possible.
"Our allies and supporters should know that this situation has little to do with organizing and employee choice, which we support, and everything to do with the method by which it happens, as well as with protecting CAIR's legal identity as an American Muslim civil rights organization.
"We hope to quickly resolve this matter and to publicly share more information as soon as possible. Again, in the meantime, please know that we strongly support a fair and just workplace for all members of our CAIR family."