The Trump International Hotel opened in Washington in October 2016. (AFP/Getty Images)

Democrats in Congress think they've found a way to shed some light on how much money President Trump stands to make through his business empire by simply being president.

With Congress out of town in August, Democrats on the House Oversight and Government Reform Committee launched an investigation Tuesday into what the federal government is spending on Trump properties.

They sent letters to the heads of 23 major federal agencies requesting that the agencies calculate how much they are spending on products or services by Trump-owned-or-affiliated business: Did your agency travel and stay in a Trump hotel? How much did you spend? Did your agency provide grants to a Trump-related hotel? How much does the Secret Service spend staying at and securing the many Trump properties he and his family stay at all over the world?

If Democrats can get this information — and it's a big if — it could be the first time someone is able to pin down how much money related to Trump's day job is going to his businesses. The federal government is the largest employer in the country. It employs nearly 3 million people, a number of whom travel often.

“Taxpayers need to know what our taxpaying dollars are being used for when it involves a question of prioritizing the United States of America or prioritizing the enrichment of the president's companies,” said Rep. Bonnie Watson Coleman (D-N.J.), who helped write the letters.

But there are major hurdles to getting that information.

As of right now, their request is just that — a request. Republicans would have to sign onto this effort to turn that request into a requirement via a subpoena. Watson Coleman says that behind closed doors, Republicans express concern about Trump's finances mingling with the presidency, but they're not willing to go public with it.

The Trump International Hotel inside the federally owned Old Post Office building in downtown D.C. has been mired in controversy even before opening its doors. (Claritza Jimenez,Osman Malik,Jonathan O'Connell/The Washington Post)

A spokeswoman for the Republican chairman of the committee, Rep. Trey Gowdy (S.C.), declined to comment. The White House did not return a request for comment.

The White House has said it doesn't have to comply with congressional requests to anyone who doesn't chair a committee. They backed down from that after Sen. Charles E. Grassley (R-Iowa) issued a tsk-tsk to the Trump administration, but it's still an open question if agencies will devote time and resources to tracking this information.

Committee Democrats said that if they don't receive information by the end of August, they will consider using a little-known committee rule that allows at least seven members to force agencies to submit to their request. At least 18 signed onto the letter.

Since day one of Trump's presidency, Democrats and watchdog groups have been trying to pin down how Trump stands to benefit financially from being president. They don't have a lot to work with, because Trump hasn't given them much to work with.

He retains an ownership stake in the Trump Organization, which has properties and provides goods and services around the world. He has passed running the business onto his sons and said there won't be any new deals while he's president. But he could talk to them anytime about how business is going.

D.C. and Maryland are suing President Trump for violating a little-known constitutional provision called "the emoluments clause." (Jenny Starrs/The Washington Post)

He won't release his tax returns — unlike every other president has for the past four decades — so we don't know how much he makes annually or whether he has any financial ties to foreign governments. There are bills in Congress that would force him to share his returns, but Trump would have to sign to make them law.

So Democrats and watchdog groups are going around Trump and using what's publicly available: what businesses are his (most of them have his name slapped across them; wine, hotels, golf resorts) and how much money the federal government spends (which the government has to keep track of).

Lawmakers have already identified a few potential conflicts. The Washington Post reported the State Department spent $15,000 at a Trump hotel in Vancouver to protect members of Trump's family as they headlined its grand opening.

When Eric Trump went to Uruguay to promote a Trump hotel and condominium tower, the Secret Service spent $9,510 for hotel rooms — and the trip cost $97,830 for taxpayers.

Eric Trump and members of his Secret Service detail walk outside La Huella, a beachfront restaurant, during a private business trip in early January to Punta del Este, Uruguay. (Cristian Cordoba)

The Post has also reported that the Defense Department was looking into renting space at Trump Tower.

Then there's the president. “President Trump also makes frequent trips to properties he owns, and these trips may result in U.S. taxpayers' money flowing into president Trump's pockets,” the letter says.

We haven't even mentioned foreign dignitaries who come to visit and stay in a Trump hotel. On that front, Democratic attorneys general in D.C. and Maryland have sued Trump directly, alleging that he is violating the Constitution by accepting gifts in the form of foreign nations staying at his D.C. Trump hotel.

Or the intangible business benefits that come from being a Trump, like deals going through.

All this worries good-government types, who say it's anathema to democracy to have a president that, at the very least, gives the appearance he is making decisions that enrich him personally.

“This does suggest that Trump is using his position as president for his personal financial benefit,” said Melanie Sloan, a board member of watchdog group American Oversight, which has requested the same information as Oversight Democrats and plans to sue the government to get it. “It's not just the Secret Service. He'll demand some secretary come there and be there two days. And he's charging for that.”

If Congress can figure out how much the federal government spends at Trump properties, it would put the first dollar amount on how Trump's presidency may be enriching his own pockets.