The Clinton campaign has tagged the Sanders agenda as overly expensive, requiring either a dramatic increase in the deficit or tax increases that hit not only the nation’s wealthiest but millions of middle-class families as well. Politically, the Sanders plan is only achievable with the kind of the Democratic majorities in the House and Senate that Obama enjoyed briefly in 2009. Clinton’s proposal, by contrast, is pegged to the reality that barring an electoral tsunami in 2016, she would have to work with at least one chamber of Congress controlled by Republicans, and maybe two.

“The plan is a promising step in the right direction,” said Brian Pallasch, chief lobbyist for the society of civil engineers. That’s a more polite way of saying it doesn’t go far enough. “Most of these plans are constrained by our ability to raise revenue,” Pallasch said. “This one is probably no different than that.”

Clinton’s allies don’t deny that. She hasn't detailed exactly how she’d pay for the plan other than to say she would raise revenue through “business tax reform.” That probably means she is looking to tap the same fiscal spigot as are Democrats and some Republicans in Congress (as well as the Obama administration)—a one-time infusion of cash from corporations that bring their profits back from overseas accounts in exchange for a tax discount. Such a move would bring in enough money to boost infrastructure spending for several years, but it would not be able to sustain those levels into the future.

A permanent fix for highway spending, for example, would require Congress to do something it hasn’t done in 22 years: increase the federal gas tax. And Clinton is not calling for that. Nor is she advocating a switch to a tax on vehicle miles traveled, an alternative being tried in Oregon that some analysts say would be a fairer way for drivers to pay for public roads. Endorsing either idea would leave Clinton vulnerable to attacks from Republicans who would call them a tax on the same “everyday Americans” she has vowed to protect.

“It would be pie-in-the-sky for Hillary to say we’re going to base the infrastructure plan on a tax increase. Because it's not getting through the Congress, no matter how right it is,” said Ed Rendell, the former Pennsylvania governor who now advocates for infrastructure improvements as a co-chairman of Building America’s Future.

Rendell, who has endorsed Clinton and consulted on her proposal, told me the plan is more ambitious than the top-line number would suggest. That’s in large part because unlike the Sanders proposal, it does not rely as heavily on direct government spending. Clinton is calling for $250 billion in new spending, plus another $25 billion that would go toward launching a national infrastructure bank, which she borrowed from an Obama administration proposal that stalled in Congress. The new government entity, combined with the reauthorization of a Build America Bonds program that was included in the 2009 stimulus, would support private investments that could bring total infrastructure spending to around half a trillion dollars, according to the campaign. “It’s bold and at the same time realistic,” Rendell said.