A standoff between port operators and the union representing East and Gulf Coast dockworkers, fueled by disagreements over wages and the use of automation, culminated in a strike on Tuesday, with tens of thousands of workers walking off the job after midnight.

The existing contract between the union, the International Longshoremen’s Association, and the port operators group, the United States Maritime Alliance, which covers roughly 45,000 longshoremen, expired on Monday. Negotiations have stalled since June, when, citing the use of labor-saving technology at the port in Mobile, Ala., the I.L.A. broke off talks.

In the final hours before Tuesday’s strikes, there were some signs of movement in negotiations for a new six-year contract. But the sides remain at odds over wage increases and the use of automated equipment, which unionized workers say could jeopardize their jobs.

“We are prepared to fight as long as necessary, to stay out on strike for whatever period of time it takes, to get the wages and protections against automation our I.L.A. members deserve,” the union’s president, Harold J. Daggett, said on Tuesday.

Here’s what we know so far about the offers on the table.

A gap in wage-increase proposals

Under the expired contract, the most senior longshoremen earn a top rate of $39 an hour. That’s up 11 percent from the start of their previous six-year contract. But over that same period, inflation has risen 24 percent. And while longshoremen with seniority can earn more than $200,000 a year with overtime, they say they have to put in long workweeks to earn that much.

The I.L.A. had been asking for a $5-an-hour raise for each of the six years of the new contract, which for the top-earning longshoremen would mean $69 an hour in the final year of the agreement, or a 77 percent total increase. But Mr. Daggett told CNBC on Tuesday that the union was now pushing for a roughly 61.5 percent raise. These offers could keep changing in the course of the negotiations.