Donald Trump and Nancy Pelosi finally agree on something, with both floating massive infrastructure spending in an expected fourth coronavirus economic relief package. But reaching a deal could again prove impossible.

Suddenly, it's "Infrastructure Week" again in Washington. Only that under a lockdown order from Mayor Muriel Bowser and warnings from Trump administration officials about Covid-19 headed to the nation's capital, the most bipartisan joke in town is not so funny this time.

The White House and lawmakers have tried this several times since Mr Trump became president. But each time, for reasons exclusively political, talks have broken down.

But there is new hope -- however scant -- that the coronavirus outbreak and the need to steel the US economy by creating new jobs might be the one force that can get the Trump White House on board with congressional Republicans and Democrats, who have offered some bipartisan plans that are gathering dust in the vacant halls of Capitol Hill.

Pelosi went first on Tuesday. During a television interview during which she also said she prefers to communicate with the TV-minded president via her public comments rather than speaking to him personally, she appeared to make her opening pitch.

"I want to be very specific, because I hear people saying, '[Democrats are] doing this wish list,' and that isn't so," she told MSNBC. "We have agreed in our negotiations that everything that we're doing is specific to the coronavirus challenge and that would be to do infrastructure for water systems that are so essential, broadband because so many people are relying on telecommunication and social media and the rest."

She also called for the so-called "phase four" coronavirus relief bill to cover "other aspects of infrastructure that will help get us through all of this." Falling short on her pledge to be "very specific," she offered up only "issues that relate to surface transportation and the rest."

Though her statement was as broad as it was vague, Ms Pelosi got Mr Trump's attention.

"With interest rates for the United States being at ZERO, this is the time to do our decades long awaited Infrastructure Bill. It should be VERY BIG & BOLD, Two Trillion Dollars, and be focused solely on jobs and rebuilding the once great infrastructure of our Country! Phase 4," the president tweeted, appearing to float a package even larger than that described by his nemesis, the Democratic speaker.

Though Ms Pelosi would likely enthusiastically embrace a massive spending package for everything from roads and bridges to broadband and water systems to tunnels and ports to airports and dams, history shows just how difficult a bill will be. Here are three reasons why.

What are we even talking about?

Ms Pelosi is considered a skilled legislator and a savvy negotiator. So it's no surprise she didn't show many cards in her Morning Joe interview.

But, as any Washington veteran would, she laid down a couple key markers.

But Mr Trump, who once published a book called The Art of the Deal, opted to talk broadly. This frustrates Democrats and Republicans alike on Capitol Hill. That's because they so rarely know exactly what he would sign into law -- sometimes until very late in the legislative process when it can be hard to put suddenly-busted deals back together.

To be sure, he has talked boldly since he was a candidate. Here was Mr Trump in January: "We want to build new roads, bridges, tunnels, highways bigger, better, faster, and we want to build them at less cost."

Right out of the gate this time, it's not immediately clear Mr Trump and Ms Pelosi are even talking about the same package.

Those dreaded 'pay-fors'

In Washington speak, "pay-fors" means cuts to other programs to pay for a set of new federal projects.

A source with knowledge of previous attempts during the Trump era to broker an infrastructure deal said the three sides key to passing such a package -- the White House, Senate Republicans and House Democrats -- agree on about 80 per cent of an infrastructure deal.

The biggest hang up?

How to pay for it, of course.

The White House last year proposed a public-private funding split that was much more private than public. It was considered so far-fetched in Washington that even Senate Republicans -- and some House GOP members -- ran away from the Trump plan as quickly as they could.

It was dead on arrival in a major embarrassment for a president who, at that time, was grasping for a big legislative win.

It's an election year

Speaking of legislative wins, including an infrastructure overhaul package in a "phase four" Covid-19 bill would amount to a victory for both parties.

With America's roads, bridges, and, as Ms Pelosi would say, "the rest," ageing and crumbling -- and with much of rural "Trump Country" in need of speedy broadband -- there is no question the idea is good politics for both sides.

That's doubly true in an election year.

It's also more unlikely in an election year.

Resolving the remaining 20 per cent of the infrastructure disagreement would be relatively easy, especially as both sides are eager to avoid economic calamity. But it would be easier in many ways for a deal to again blow up, allowing each side to raise campaign cash and fire up their base by blaming the other side.

Recall the last time they tried this. It ended in Mr Trump erupting in the White House Situation Room and leaving a meeting with congressional leaders.

That gave Ms Pelosi all the political ammunition she needed to cast doubt on Mr Trump's talk during his February State of the Union Address about trying again.

"President Trump finally sat down at the table with congressional Democrats to craft a bipartisan infrastructure proposal that would revolutionize our nation's infrastructure," she said in a 4 February statement.