JUPITER, Fla. — On Monday, minutes before 10 a.m., members of the negotiating team from the Major League Baseball players’ union made their daily walk from the parking lot of Roger Dean Stadium into their meeting room. Among the group: the veteran pitchers Andrew Miller and Max Scherzer.

An hour later, a few M.L.B. officials walked over to the union’s side of the facility. A full day of talks are expected, and time is of the essence.

M.L.B.’s regular season is scheduled to start March 31. But after making little progress toward a new labor deal with the union last week, the league doubled down on its position with a threat.

M.L.B., which locked out the players on Dec. 2, told the union on Wednesday that it was serious about its self-imposed deadline, Monday, for reaching a new collective bargaining agreement in order to begin the 162-game season as scheduled. If that deadline was not met, the league said, it would begin canceling games, not pay players for those missed games and not reschedule them.

Although the union didn’t agree with M.L.B.’s deadline, it understood the gravity of the situation and was willing to explore every avenue to strike a deal in which a full season could be played.

On Sunday, the sides laid some of the groundwork for the final day. They did not exchange any formal proposals but had discussions and floated ideas. M.L.B. described the day as productive. The union’s assessment was more muted given the hurdles that remained.

And now the baseball world, already frozen for months, waits for either a conclusion — or an extension. Will the 11th hour spur compromise, or will the sides remain entrenched?

In early February, Commissioner Rob Manfred said losing regular-season games would be “disastrous” for the industry. Manfred is employed by the owners but is tasked with being a steward of the game. Will he cancel games after saying it would be bad for everyone? Or will M.L.B.’s stance soften in the final hours before the deadline if progress is made?

While it has come entirely during the off-season, this lockout qualifies as the second-longest work stoppage in baseball history. The longest was the 1994-95 players’ strike, which resulted in the loss of more than 900 games and canceled the 1994 World Series.

The current labor negotiations weren’t expected to be easy. The past two collective bargaining agreements were viewed as having further tilted the balance in the owners’ favor. Thus, when one side wants to make significant changes to the system — the talent on the field — negotiating can be tense and full of brinkmanship. The union has been growing its rainy day fund for years for this very fight against M.L.B. owners, who ran an $11 billion-a-year business before the coronavirus pandemic.

While top players have been awarded record contracts in recent years, including this off-season, before the lockout, players are frustrated that their average salary of roughly $4 million has reached a plateau and has not kept pace with team revenues. Most players in 2021 had salaries under $1 million. And the average career length is about four years.

So the union has sought a series of changes, such as obtaining earlier compensation for younger (cheaper and more relied upon) players, improving competition among teams, curbing service time manipulation and injecting more spending.

The league, though, believes the players have a fair system without a hard salary cap and sees the issue as a matter of wealth distribution — that star players are disproportionately commanding more money than others. While owners have listened to the union and offered ways to pay younger players more, they have also proposed some ways to generate more revenue to pay for it (such as expanding the playoffs or putting advertising patches on uniforms) and some methods to curb spending elsewhere (such as their stricter luxury tax model).

On Saturday, the union made its largest set of moves to date in these negotiations, with its offer coming on the day spring training games had been originally scheduled to begin. The union significantly lowered its asks on expanding salary arbitration eligibility and changes to the revenue sharing system, and made tweaks in the league’s direction on the luxury tax.

But M.L.B., which has insisted that salary arbitration and revenue sharing are third-rail topics, and thus any changes to them are not concessions, thought the union did not go far enough in that proposal. And when the league’s negotiating team conveyed that sentiment, players were furious and considered walking away from talks.

They didn’t, and the union’s negotiating team remained in Jupiter through Monday. The sides have met every day at the unused spring training home of the St. Louis Cardinals and Miami Marlins in hopes of making progress. Neither spring training nor its exhibition games have begun on time, and now the start of the regular season is in jeopardy.

M.L.B. chose Monday as a deadline because it believed a minimum spring training length of four weeks — two weeks shorter than normal — made sense to avoid a spike in injuries like the one before the pandemic-shortened, 60-game 2020 regular season.

As a counterbalance to M.L.B.’s deadline threat, the players previously issued their own threat during these labor negotiations: They told the league they were reluctant to grant club owners an expanded playoffs — worth an estimated $100 million annually — if games and money were taken from them.

Among the big items that remained unresolved entering Monday: They remained separated on the league minimum salary ($640,000 versus $775,000), a bonus pool for top players not yet eligible for salary arbitration ($20 million for 30 players versus $115 million for 150 players), salary arbitration expansion (22 percent of players with between two and three years of service time versus 35 percent), and the luxury tax and revenue sharing systems.

As they have in past labor negotiations ahead of a deadline, the sides could work deep into the night and the next morning on a new deal if Monday’s talks prove fruitful. Or if they cannot make enough headway, they could call off talks for now, and resume them later this week. After nearly a year of discussing a new C.B.A., though, the season and its money are on the line now.