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Promising a trillion dollars of infrastructure investment was an easy win for a billionaire builder running for the White House: Roads and bridges need fixing, workers need jobs, and the proposal has bipartisan appeal.

Accomplishing it as president? Not so easy.

President Donald Trump faces high stakes — some in his own party oppose more government spending — while others see potential political benefits and more: He could unite Republicans and Democrats on a key issue and big investment could contribute to the millions of jobs he promised as a candidate.

Trump's just got to answer the trillion-dollar question that’s plagued decades of infrastructure attempts in the past: Who pays?

As a candidate, Trump promised it wouldn't be taxpayers. He vowed to put together a trillion dollars of investment over 10 years through tax cuts that incentivize private companies to invest in ways that they can later use to make money — like if a company repaired a road for the right to charge tolls on it.

"It’s saying basically, we’re only going to fund profitable projects...It’s not clear that the campaign plan would fund anything that wasn’t already getting done," said Ron Klain, the former Obama White House aide who oversaw implementation of the last major infrastructure effort, the 2009 stimulus bill.

Related: Keeping Trump's promises will be challenging

"Our most urgent infrastructure needs are things like water systems in Flint, Michigan," he added. "They’re public goods — we call them that because the private sector doesn’t want to do them."

Trump's team says new tax revenues would pay for the projects — a view some economists don’t share. The White House made it clear that private-sector investment is a key part of their plans when they tapped a former public-private partnership financier from Macquarie Capital, DJ Gribbin, as the newly appointed special assistant to the president on infrastructure.

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“To launch our national rebuilding, I will be asking Congress to approve legislation that produces a $1 trillion investment in infrastructure of the United States — financed through both public and private capital — creating millions of new jobs,” Trump said during his joint address to Congress last month.

House Speaker Paul Ryan said the bill could come as early as this spring, as part of a budget deal, but already there's reporting that it could get pushed back to 2018 as Republicans tackle tax reform and the stumbling health care replacement.

Trump's trillion-dollar goal is less than a quarter of the $4.59 trillion the American Society of Civil Engineers estimates is needed to bring the nation’s infrastructure system from this year’s near-failing D+ to B grade.

Related: Infrastructure promises come due

Their assessment was dire: One out of every five miles of highway pavement is in poor condition; there are 15,500 “high-hazard potential” dams whose failure could kill people; and there’s a $90 billion repair backlog in America’s mass transit systems. The group predicts that not addressing the “investment gap” could cost the nation $4 trillion in GPD and the loss of 2.5 million jobs in 2025.

Democrats have been clamoring to dig in on the issue for years — the last big infrastructure investment came in President Barack Obama’s 2009 stimulus, though a smaller $305 billion bill was signed into law in late 2015.

Congressional Democrats made a big show of introducing a trillion dollar infrastructure bill early this year aimed at offering a solution to one of Trump's top priorities. They say the public-private partnership — called "P3s" — can work for some big-ticket projects, but not all. They also argue it favor corporations over consumers.

“You get a tax cut [to repair a road] and then you get to toll it for a hundred years? I’ll quit Congress and start a P3,” Rep. Peter DeFazio, ranking member of the House Transportation and Infrastructure Committee and Oregon Democrat, told NBC News.

“As I said to one of Trump’s advisors, I think a lot of your rural voters would not be pleased that they’re paying gas taxes, they’re paying federal taxes, and now, by the way, you’re paying a toll to a private entity to repair your bridges and roads,” he added.

Related: Senate Democrats propose $1 trillion infrastructure plan

DeFazio said it’s a “fallacy” to think that public-private partnerships can upgrade and repair the nation’s infrastructure. He pointed to a bipartisan 2014 term-long study which found that P3s “have the potential to address only a small portion of the nation’s infrastructure needs.”

He said there’s Congressional appetite for action, but he doesn’t know how committed the president is, particularly in the face of opposition from some in the GOP.

Government spending just isn't an option for Speaker Ryan and the GOP's fiscal hawks. Ryan has said the GOP is OK with an infrastructure bill if and only if all the tax credits and public funding portions of the deals can be fully offset with cuts elsewhere.

On the Charlie Rose Show, he painted a picture of public-private partnership on infrastructure spending.

“For every one dollar of federal dollars, there's $40 of private sector spending,” Ryan said. “We want to leverage as much private-sector dollars as possible to maximize the fixing of our infrastructure.”

Still, conservative groups are cautioning Republicans to tread lightly.

The Heritage Foundation's infrastructure researcher, Michael Sargent, told NBC News that he "was not thrilled when spending a trillion dollars on infrastructure became a central tenant on the campaign trail...On the fiscal side of things, it does not line up with conservative priorities."