Major League Baseball quickly learned Monday the challenges it faces on multiple fronts in its desperate effort to launch its 2020 season. After MLB outlined a plan on a tentative midsummer start to the 30 team owners in a conference call Monday, the players union rejected the economics of the proposal before even receiving it.

MLB’s preferred plan is to hold a streamlined “spring training 2.0” in June, then stage 78 to 82 regular season games beginning in July — without fans, at least at first — in as many home stadiums as possible. Some teams, if necessary, would relocate their operations to spring training facilities in Arizona or Florida. Teams would be grouped regionally, as opposed to by league and division, to reduce travel considerations. Any such path forward, however, requires a handful of assumptions on the availability of testing and states reopening their economies, none of which can be regarded as certain.

But while much of that plotting and best-case projecting remains dependent upon the unknowable trajectory of the novel coronavirus pandemic — which has delayed Opening Day by more than six weeks and counting — the thorny matter of how players will be paid, proceeding on a parallel track, is already off to a bad start.

The union is signaling firm opposition to MLB’s idea, outlined in the proposal it will present Tuesday, for a roughly ­50-50 split of revenue in 2020. Unlike sports with salary caps, MLB does not have a revenue-sharing system with players, and the adoption of one would be an unprecedented step for the sport.

The union equates a defined revenue-sharing plan as a de facto salary cap and considers MLB’s proposal to be a non-starter. “A system that restricts player pay based on revenues is a salary cap, period,” union chief Tony Clark told the Athletic on Monday. He further accused MLB of “trying to take advantage of a global health crisis to get what they’ve failed to achieve in the past.”

An MLB official denied the proposed revenue share constitutes a salary cap but said it rather is a one-year-only means of sharing money under difficult circumstances that make it impossible to project what revenue will be.

The union also contends the matter of player compensation for 2020 was already decided in the agreement the sides reached March 26 — which was supposed to have been Opening Day — to govern the conditions of the shutdown. As part of that agreement, players received a $170 million advance on their 2020 salaries and agreed to be paid prorated portions of their remaining salaries based upon the number of games played.

However, MLB contends the March agreement pertained to games played with fans and that the notion of games played without them requires a different calculus to account for the loss of revenue from tickets and other game-related income, such as parking, concessions and luxury suites. Both sides appear dug in on their positions; baseball’s long, miserable history of labor wars suggests an ugly battle looms.

MLB, with industry revenue of $10.7 billion in 2019 according to Forbes, contends it will lose billions this year regardless of what happens from here on and that it will lose money with every game played without fans unless it gets additional salary relief from the players. The union believes that viewpoint fails to account for the reduction in expenses from fan-free games, as well as the potential windfall for owners from an ­expanded postseason amid a heightened appetite for live sports on the part of fans.

MLB has undertaken several cost-saving measures, including reducing the 2020 draft from 40 to five rounds and granting teams the leeway to lay off or furlough employees.

The revenue-sharing proposal could spark a bitter reaction among players — who see themselves, along with other on-field personnel, as the group being asked to take on the greatest health risk in any plan to bring back baseball before a vaccine arrives. That risk would be underscored by the empty stadiums: If having fans is seen as too risky, what does that say about the personnel on the field — particularly coaches and umpires, who tend to be older?

“It feels like we’ve zoomed past the most important aspect of any MLB restart plan: health protections for players, families, staff, stadium workers and the workforce it would require to resume a season,” Washington Nationals reliever Sean Doolittle wrote Monday on Twitter. “We need to consider what level of risk we’re willing to assume.”

Union leaders point out that owners have time to recoup their losses — most of them remain in the game for decades and have seen their franchises increase greatly in value in recent years — but players have a finite window of their athletic primes in which to earn their money.

“Players lose money for every game not played,” one union person said, “and will never get it back.”

It is within this atmosphere of fundamental distrust and historical rancor that baseball must figure out a way forward. And increasingly, it isn’t so much the coronavirus that looms as the biggest existential threat — it’s the money.