The federal government is on track to give $6.88 billion, the most ever awarded to a mass-transit project, for the construction of a second rail tunnel under the Hudson River to New York City, Senator Chuck Schumer said Wednesday.

Mr. Schumer, the Democratic majority leader from Brooklyn, said he intended to announce the grant in the city on Thursday. A White House aide confirmed that the Department of Transportation planned to notify the tunnel project’s sponsor, the Gateway Development Commission, of the decision this week.

The two-tube tunnel is part of Gateway, a massive infrastructure project that is widely considered the most important in the country. The new tunnel would supplement a troublesome pair of single-track tunnels that opened in 1910 and have been steadily deteriorating since Hurricane Sandy flooded them with salt water in 2012.

The federal pledge will allow Gateway’s planners to start seeking companies to construct a tunnel parallel to the deteriorating ones, a project that is expected to cost more than $16 billion before it is completed in 2035. Gateway’s planners still hope to receive more funding from other federal programs to raise Washington’s share of the total cost to at least half.

The governors of New York and New Jersey agreed last year to an even split of the local share of the cost of building the tunnel. That agreement was a critical precursor to obtaining federal funding for the project. But a signed deal with the federal government is not expected until early next year.

Regional transportation officials have been in a hurry to secure a federal commitment to Gateway while President Biden is in office and Democrats have control of the Senate. Gateway is a pet project of Mr. Schumer’s. Mr. Biden, a longtime rider of Amtrak trains between Washington and his home state of Delaware, has been an ardent supporter.

“This was the major hurdle, getting this kind of very large investment from the federal government, and here it is,” Mr. Schumer said. “This is real, and it means there’s no turning back now.”

Politics aside, transportation officials say that the tunnel is urgently needed because the existing tunnels must be cured of the lingering effects of their inundation by Hurricane Sandy. Amtrak, which owns the tunnels, plans to shut those tracks for repairs, one at a time, once the new tunnel is in use.

A few years ago, commuter trains arriving at Pennsylvania Station in Manhattan from New Jersey were jam-packed with passengers standing in the aisles and vestibules. The pandemic sharply reduced the number of commuters, many of whom are not expected to return to offices five days a week. Still, analysts at the Regional Plan Association forecast last year that “trans-Hudson travel demand on the heaviest travel days is likely to be at or above pre-Covid levels” by the time the new tunnel is completed.

If one of the existing tracks had to be closed before the new tunnel is available, train capacity between Penn Station and places west of the Hudson would be cut by 75 percent during rush hours, according to Amtrak. Mr. Schumer and other elected officials said that reduction would sharply curtail commercial activity along the East Coast and could devastate the American economy.

“The Gateway program is an essential economic engine for New York and the nation,” said Kathy Hochul, the Democratic governor of New York, in a statement.

Philip D. Murphy, the Democratic governor of New Jersey, said in a statement that he would work with officials in Washington to secure “the remaining funding necessary to complete the most urgent infrastructure project in the country.”

Beleaguered commuters in the New York region may think they have heard this all before. Two decades ago, a different project to build a train tunnel under the Hudson received a $3 billion commitment from the Federal Transit Administration.

Construction of that tunnel, known as ARC, began in 2009. But a year later, Chris Christie, then the Republican governor of New Jersey, canceled the $8.7 billion project, saying that his state could not afford the potential cost overruns. Afterward, New Jersey repaid $95 million of the $271 million the federal government had provided.

Kris Kolluri, chief executive of the Gateway Development Commission, said he was confident that construction of the tunnel would begin next year and that there would be no stopping it this time.

“I have never seen this level of cooperation,” he said. “There’s no daylight between the state, local and federal partners.”

In October, the commission asked the Federal Transit Administration to provide $6.65 billion through its Capital Investment Grants program. Mr. Schumer said the grant exceeds that request by more than $200 million because construction costs have been rising steadily.

Mr. Kolluri said he believed the commission’s estimated cost of $16.1 billion was solid, even though the transit agency is already signaling that the cost could rise to $17 billion. The commission is still awaiting a response to its request for $3.8 billion from a separate pot of federal money.