As top Democrats meet with President Donald Trump at the White House on Tuesday to discuss possibly forging a bipartisan infrastructure package, the underlying issue that's derailed big road and bridge bills over the past ten years remains an unsolved political riddle, and that's how the feds pay for hundreds of billions of dollars in additional highway construction.

"We look forward to hearing your ideas on how to pay for this package to ensure that it is big and bold enough to meet our country's needs," top Democrats told the President in a Monday letter.

That letter didn't mention that Democrats haven't identified how they would pay for their plan, either - or how big it would be, reportedly somewhere in the range of $1-to-$2 trillion.

At the White House, there has been the same type of mushiness when it comes to funding details.

"The President’s target of $1 trillion in infrastructure investment will be funded through a combination of new Federal funding, incentivized non-Federal funding, and newly prioritized and expedited projects," officials wrote last year, as the acting White House budget chief said earlier this year that Congress should fill in the blanks.

"It continues to be a major priority of this administration," acting Office of Management and Budget chief Russ Vought told a House hearing last month.

But Democrats were having none of it.

"We've gone over two years and all we have is the same one page description of the President's plan that we had this time last year," said Rep. Lloyd Doggett (D-TX).

A very similar political dance over infrastructure went on during the Obama Administration; in President Obama's 2012 budget, he proposed spending $328 billion extra over ten years on new roads and bridges - but officials never spelled out for Congress the source of the new funding - and Republicans never acted on the request.

Some veteran transportation officials aren't sure this effort by President Trump is going to be any different.

"I am one person very skeptical about any major infrastructure program," said Emil Frankel, a former Connecticut Department of Transportation Commissioner, who was also appointed to serve in the federal Transportation Department under President George W. Bush.

"First of all the atmosphere is such, that it seems to me impossible to imagine any agreement across party lines," Frankel told a Capitol Hill event last week hosted by the Information Technology and Innovation Foundation.

National business and transportation industry groups have pushed for years to have Congress spend more money on infrastructure - and they continue to make that point this week.

"This is a great opportunity to get to work on modernizing America’s infrastructure," the U.S. Chamber of Commerce tweeted on Monday.

But it still all comes down to money - and with the yearly deficit expected to be over $1 trillion in 2019, there's not a lot of extra cash just sitting on the shelf, waiting to be turned into highway concrete.

And a quick internet sample of headlines from around the country shows right now might not be the best time to increase gas taxes.

"Gas prices rising in Syracuse, across Upstate New York," read one.

"Tuscon gas prices rising," one TV station reported in Arizona.

"Gas prices are still rising," a Florida TV station noted.

"Historically, the gas tax has provided adequate funding, but the increasing number of electric vehicles and hybrids as well as fuel tax evasion makes the gas tax an increasingly inefficient funding mechanism," argued Baruch Feigenbaum and Austill Stuart of the Reason Foundation earlier this year.

If a gas tax increase is out of the mix, the other options to fund an infrastructure package might include:

+ Shifting money from general revenues into new roads and bridges;

+ Allowing states the option to levy tolls on older interstate highways;

+ A financing route involving the private sector;

+ Or moving to a futuristic system known as "Vehicle Miles Traveled," where a transponder would track the distance individual vehicles go, and motorists would be charged accordingly.

The federal gasoline tax has not increased since 1993; experts say that means the inflation adjusted value of that revenue has gone down by close to 50 percent in the last 26 years.