WASHINGTON (NYTIMES) - A lawsuit accusing US President Donald Trump of violating the Constitution by maintaining a financial interest in his company's Washington hotel cleared a critical hurdle on Wednesday (July 25) when a federal judge allowed the case to move forward.

In the first judicial opinion to define how the meaning of the Constitution's anti-corruption clauses should apply to a President, Judge Peter Messitte of the US District Court in Greenbelt, Maryland, said the framers' language should be broadly construed as an effort to protect against influence-peddling by state and foreign governments.

He ruled that the lawsuit should proceed to the evidence-gathering stage, which could clear the way for an examination of financial records that the President has consistently refused to disclose.

The Justice Department is expected to forestall that by seeking an emergency stay and appealing the ruling.

The two constitutional clauses at issue restrict a President's ability to accept financial benefits or "emoluments" from domestic or foreign governments, other than his official salary.

The plaintiffs in the lawsuit, the District of Columbia and the state of Maryland, say that Mr Trump is violating those bans by accepting profits from the Trump International Hotel, a five-star hotel just blocks from the White House that is frequented by foreign and state officials.

The judge earlier ruled that the local jurisdictions had standing to sue because the Trump hotel arguably siphons off business from their convention centres or hotels.

Mr Trump has resisted efforts to force him to provide more details about his personal finances, and in particular has refused to release his tax returns.

Questions about Mr Trump's possible business dealings in Russia or with Russians have come up in connection with the inquiry being conducted by the special counsel, Mr Robert Mueller.

Before taking the oath of office, Mr Trump resigned his role in his company, the Trump Organisation, but retained his ownership and turned management over to his two oldest sons, Eric and Donald Jr.

The Justice Department had sought to dismiss the case, arguing that the constitutional restrictions do not apply to Mr Trump's interest in the hotel. But the judge said the department's lawyers were defining an emolument far too narrowly.

"Sole or substantial ownership of a business that receives hundreds of thousands or millions of dollars a year in revenue from one of its hotel properties where foreign and domestic governments are known to stay (often with the express purpose of cultivating the President's good graces) most definitely raises the potential for undue influence, and would be well within the contemplation of the clauses," he wrote in a 52-page opinion.

He said the Justice Department was trying to equate an emolument with a bribe from foreign or state government officials. But the Constitution already defines bribery as an impeachable offence, and bribery is extremely difficult to prove, he wrote.

The weight of historical evidence shows that the framers meant the emoluments clauses to act as a broader check on influence-peddling that could influence a President's decisions, the judge said.

"Where, for example, a President maintains a premier hotel property that generates millions of dollars a year in profits, how likely is it that he will not be swayed, whether consciously or subconsciously, in any or all of his dealings with foreign or domestic governments that might choose to spend large sums of money at that hotel property?" the judge asked in his opinion.

"How, indeed, could it ever be proven, in a given case, that he had actually been influenced by the payments?" he added. "The framers of the clauses made it simple. Ban the offerings altogether."

Mr Andy Reuss, a Justice Department spokesman, said lawyers there "continue to maintain that this case should be dismissed".

He said that a federal judge in New York had dismissed a similar lawsuit brought by different plaintiffs.

"The department is reviewing the order and determining next steps to continue vigorously defending the President," he said.

Lawyers for the plaintiffs cheered the ruling.

"The decision is extremely important, said Mr Norman Eisen, who is the chairman of the Citizens for Responsibility and Ethics in Washington and acts as co-counsel for the plaintiffs.

"It represents a major leap forward in being able to understand how Trump is profiting off the presidency."

Judge Messitte seemed especially struck by one example cited by the plaintiffs. He noted that when Mr Paul LePage, the Republican governor of Maine, visited Washington to meet with Mr Trump on official business in February 2017, he and his staff stayed at the Trump International Hotel.

At a subsequent news conference with the governor, the President announced that he was reviewing decisions related to national monuments and parks that were adopted the year before by the Obama administration and opposed by the governor.

The governor's spokeswoman has said that Mr LePage did not choose the Trump hotel in an attempt to please the President.

In a footnote, the judge also noted that the Trump International Hotel was reportedly one of the best performing properties run by the Trump Organisation, reaping more than US$40 million (S$54.33 million) in revenue in 2017.

He had earlier ordered that a wide range of documents relevant to the case be preserved, including any records that would reveal the identities of Trump hotel and restaurant guests, the amounts they paid for rooms and event space, and the profits for the Trump Organisation, as well as the hotel's marketing campaigns aimed at foreign diplomats.