As the equal pay lawsuit between the United States women's national team and the U.S. Soccer Federation enters its next phase, representatives from the U.S. men's team have entered the fray, accusing U.S. Soccer of treating the women as "second-class citizens" and sending "a corrosive public message to women and girls."

The incendiary amicus brief signed by representatives of the men's players' union was one of several filed Friday in support of the USWNT's appeal in their wage discrimination lawsuit. A judge last year dismissed the USWNT's lawsuit, ruling the women had actually been paid more than the men, but lawyers for the team appealed last week, saying the dismissal was "legally wrong" and "defies reality" because it ignored the effect of the USWNT's higher win rate on the players' compensation.

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The men's union backed up the USWNT's appeal, writing in Friday's brief that U.S. Soccer "has spent more than three decades treating the women as an afterthought, discriminating against them through inferior wages and working conditions, and forcing the women to struggle for the equal pay and fair treatment they deserve."

This pattern, the brief continues, "sends a corrosive public message to women and girls that, even at the highest level, no matter how hard they work or how much they succeed, they can and will be diminished and undervalued by their employers. That is as dispiriting as it is unlawful."

A statement from the USSF said: "U.S. Soccer is firmly committed to equal pay, just as the members of our senior national teams are. We will continue working with both our Men's and Women's National Teams to equalize FIFA prize money and to chart a positive path forward to grow the game both here at home and around the world."

Although the union for the U.S. men's team has backed up the USWNT's allegations of unequal pay previously in statements to the media, this is its first time directly getting involved with the USWNT's lawsuit. Mark S. Levinstein, the executive director of the men's union, and attorney Zachary D. Tripp signed the brief.

In it, the men's union says it has "unique insight into the history of collective bargaining negotiations" between the federation and its teams, and the union "also has a keen understanding of the Federation's long-standing discrimination against the Women's National Team and its players."

The brief in particular takes issue with the district court judge's determination that the women had been paid more than the men, and it asks the appellate court to take the issue to trial. The judge last year accepted analysis from U.S. Soccer showing the men were paid $212,639 per game and the women were paid $220,747. That analysis was based on taking the total compensation paid to both teams and dividing it by the number of games each team played.

The men's union brief argues that "the district court's oversimplified math made the women victims both of their own success and of the men's atypical struggles in 2017-2018." That is because, during the time period covered in the lawsuit, 2015-2019, the women won two World Cups while the men failed to qualify for a World Cup for the first time in more than 30 years, which entitled the women to bigger performance bonuses. The court reached its conclusion "without distinguishing between appearance fees and performance bonuses," the men's union argues.

The win bonuses available to the women were lower than the bonuses available to the men in almost every scenario, according to both the men's union and the USWNT's appeal.

"A woman's rate of pay is not equal to a man's if the woman must consistently achieve better outcomes merely to get to the same place," the brief says. "If the women had won fewer games, or if the district court had analyzed a more representative period of the men's performance as a point of comparison, the per-game disparity would have been obvious, glaring, and undeniable."

This is a similar argument as the USWNT's legal team presented last week in its appeal. There, the USWNT's lawyers used the analogy of a man and woman who work the same job and are paid via an hourly wage and a sales commission. It would not be equal, they argued, to pay the woman a lower wage or a lower commission and then expect her to work more hours or make more sales to earn equal total compensation to her male counterpart.

The judge, in dismissing the USWNT's case, had also ruled that the women's team and the men's team had different compensation structures, which the players collectively bargained for, so the two contracts can't be compared.

According to their contracts, the men on the USMNT are paid for appearances and caps -- that is, when a player is called in to camp or plays -- plus bonuses when they win. The women, however, are paid through a hybrid structure where some players are paid based on call-ups and caps, but some regular players are paid salaries regardless of making rosters, in addition to win bonuses.

"The WNT explicitly rejected the terms they now seek to retroactively impose of themselves," the judge wrote in his ruling. The USWNT, in their appeal last week, pushed back on that, arguing that the judge was wrong: The women had been offered the same contract structure as the men, but never the same dollar amounts.

The men's union brief backs up the USWNT's claims that it has a different contract than the men's team because U.S. Soccer forced the women to negotiate in that direction. "The district court's analysis of the collective bargaining process was flawed through and through," the brief states.

The men's union argues that the women are not able to shop their services to other employers -- there is only one federation they can play for -- and thus the women were forced into "an untenable bargaining position" when U.S. Soccer refused to offer the women the exact same contract as the men.

"By dismissing the women's claims of discriminatory pay as nothing more than a bargained-for result, the district court endorsed the Federation's long-running abuse of its total control over athletes' ability to represent the United States to impose unfair compensation arrangements," the brief states. That "enormous power imbalance" is why the USWNT's contract can violate discrimination laws, even if it went through a collective bargaining process, the brief adds.

The brief also alleges that, instead of paying the women equally, U.S. Soccer instead "devoted substantial revenue to litigating and lobbying, in the misguided hope of preserving a system that treats the women as inferior."

U.S. Soccer has disputed that it discriminates against the USWNT, and it issued a statement after last week's appeal saying the federation "is committed to equal pay and to ensuring that our Women's National Team remains the best in the world."

The USWNT reached the semfinals at the Tokyo Olympics on Friday, beating Netherlands 4-2 on penalties to set up a showdown with Canada.