On an average day, 1,000 workers head to dozens of construction sites spread over 119 miles across California’s vast Central Valley.

Their task is monumental: Build the bridges and crossings designed to carry bullet trains that will form the backbone of a $105 billion, 500-mile, high-speed rail system whose scale has drawn comparisons to the construction of the interstate highway system.

Of course, 14 years after voters approved a nearly $10 billion bond to start building the rail system that would whisk riders from Los Angeles to San Francisco at speeds of more than 200 miles per hour, many California residents have long since lost track of what is being built where, and when or if it will ever be completed.

But if, as President Biden said in his State of the Union address, the nation is now entering an “infrastructure decade,” there is no more dramatic testing ground — or more cautionary spectacle — than California’s high-speed rail plan.

In 2008, when the bond measure passed, the project symbolized the state’s ambition to build and think big. But in the years since then, the project has become something else: an alarming vision of a nation that seems incapable of completing the transformative projects necessary to confront 21st century challenges. The rail’s planned route and scope have changed as a result of ballooning costs, political squabbles and legal challenges.

“We just have a fundamental problem in the United States of building large projects,” said Yonah Freemark, a researcher with the Urban Institute who has been following the rail plan for more than a decade. “And California’s high-speed rail is the largest of the projects.”

Never have the cases for and against the effort been so divergent.

The passage of Mr. Biden’s $1 trillion infrastructure package, astronomical gas prices and California’s insistence that the state lead the nation in addressing climate change make the moment seem perfect for providing oxygen to the plan.

But an eye-popping price tag and fundamental questions about political support are creating a critical juncture for either achieving the project’s full vision or leaving it in an expensive limbo.

“The cost of indecision on these projects is enormous,” said Eric Eidlin, a scholar with the Mineta Transportation Institute at San Jose State University who has consulted on station planning efforts for the California High-Speed Rail Authority.

Proponents say the project has always been much more than a train. If completed, they say, the system would be an economic super charger connecting two of the nation’s biggest population centers and a desperately needed alternative to choked freeways and jammed airports as climate change becomes an ever urgent challenge.

“We are the fifth largest economy in the world, and therefore I think we have to figure out how to do it,” said Arnold Schwarzenegger, a Republican who as governor championed the 2008 bond measure. “Failure’s not an option here.”

Bent Flyvbjerg, a professor at Oxford University and the IT University of Copenhagen who has studied high-speed rail projects around the world, said that such projects nearly always cost much more and take much longer to build than initially projected.

The difference between high-speed rail projects that limp along for decades and those that start running trains isn’t money, he said. It’s political energy

“The money will be found if the political will is there,” he said.

But political will within California has ebbed as patience among leaders has worn thin. The most significant turning point was the announcement three years ago by Gov. Gavin Newsom in his first State of the State address that California would start operating a truncated section of the route that would run from Bakersfield to Merced in the state’s largely rural Central Valley.

That stunned supporters and fueled critics who believed he was publicly announcing the full project’s demise, although Mr. Newsom later said the change in priority wasn’t meant to preclude finishing the full route.

Some state lawmakers, Republicans and Democrats alike, now say the effort has become flawed and unwieldy, perhaps beyond saving. Critics say that rail officials are seeking a blank check from state coffers, and that their timeline for completion is stretching unaccountably into the future.

“The project is by all objective measures in distress,” said Anthony Rendon, California Assembly Speaker, a Democrat. “Connecting the two largest urban areas in the state is the best thing we can do from an environmental standpoint and an economic development standpoint. To link two cities in the Central Valley would doom the project.”

Instead of dedicating $4.2 billion of bond money in this year’s budget to, as Mr. Newsom put it, “finish the job in the Central Valley,” Mr. Rendon said he has asked the governor to withhold funds from the project and spend more on improving existing transit systems, particularly in the Los Angeles area, which includes his district.

“What we’re focused on is building ridership for an eventual high-speed rail project, and the way you do that is by working on the bookends,” he said.

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In a recent interview, Mr. Newsom said his decision to prioritize the Central Valley segment was based on the calculation that the prospects for the full project were best if some part of it were operating.

“The pivot was never to abandon the vision,” he said. “The long term is still there.”

He added that this year’s budget proposal includes money to continue environmental and design work for the extensions beyond the Central Valley. “But it requires federal resources — not exclusively, but primarily,” he said.

A report by the California legislative analyst’s office notes that while the state’s legislature could decide to extend funding for the project — including a portion of cap-and-trade revenues through 2030 — it’s unclear where the money will come from to build beyond the Central Valley segment.

Experts say that the fragmented nature of transportation planning in the country has made the federal government hesitant to bet big on new projects rather than on fixing existing systems. That’s layered over a national political environment in which the appearance of California boosterism can be a liability, even for Democrats like the president.