By Andrew Chung and Dan Levine

(Reuters) - Legal advocacy groups seeking to challenge President Donald Trump in court over alleged conflicts of interest said filing lawsuits is part of a larger strategy to highlight their concerns and put political pressure on the White House.

In a lawsuit on Monday, ethics watchdog Citizens for Responsibility and Ethics in Washington contended payments to Trump's businesses for hotel rooms and office leases run afoul of the emoluments clause of the Constitution, which forbids U.S. officeholders from accepting various gifts from foreign governments without congressional approval.

Trump on Monday called the lawsuit "without merit," and legal experts have noted significant barriers to success in court, such as whether the plaintiffs have standing to sue.

The American Civil Liberties Union is also preparing a lawsuit over Trump's alleged violations of the emoluments clause, said ACLU Executive Director Anthony Romero. Beyond winning favorable rulings, cases can serve to "gum up the machinery of the government and rob the Trump administration of momentum," he said.

Lawsuits can also serve to frame the public debate and gain the attention of Congress and government officials, Romero said.

"This should be an area where we let a thousand legal flowers bloom," he told Reuters on Tuesday.

Laurence Tribe, one of the attorneys representing CREW, said the purpose of its lawsuit was to force Trump to abide by the Constitution, and that he felt confident they would achieve a favorable outcome in court.

He also said bringing the case can accomplish other goals.

"One of the things that this lawsuit will achieve is increased public understanding of what the issue is," he said.

Aside from the emoluments clause, conflict of interest laws that govern the executive branch generally do not apply to the president or vice president, said Erwin Chemerinsky, dean of UC Irvine School of Law and another of CREW's lawyers. Presidents largely enjoy immunity from lawsuits arising from their official duties.

The Office of Government Ethics sets policy for executive branch employees, but it does not investigate complaints or bring lawsuits. If OGE officials find evidence of wrongdoing, they refer it to the Department of Justice for civil or criminal prosecution.  

CREW also filed a complaint with the General Services Administration over Trump's hotel in Washington D.C., which he leases from the federal government. The complaint alleges the business violates the terms of the lease, which says it cannot be held by an elected officeholder. It is unclear whether that issue will ultimately be litigated in the courts.

Federal courts maintain strict rules about who has the legal right, or standing, to sue. The U.S. Department of Justice will likely raise a standing challenge to CREW's case in the form of a motion to dismiss, said Deepak Gupta, another attorney representing the group.

A spokeswoman for the Department of Justice said it is reviewing the complaint and would respond as appropriate.

In its lawsuit, CREW argued that it should be allowed to bring the emoluments case because it has been forced to spend money and divert resources away from its traditional agenda of tracking campaign contributions and ethics monitoring. Legal experts have said that will be a difficult argument to win.

ADDITIONAL PLAINTIFFS

Tribe, a professor at Harvard Law School, said CREW felt confident it would win on standing. He also noted that additional plaintiffs "have expressed interest in bringing litigation, either as part of our case or others." He declined to identify them or discuss strategy details.

Romero also said the ACLU is seeking a "suitable plaintiff."

Andy Grewal, a professor at the University of Iowa College of Law, said that adding a plaintiff with more concrete allegations, such as a hotel that claims it has lost business to Trump's hotels, would still be an uphill battle.

It would be hard to prove, for instance, that a diplomat would have stayed at another hotel instead of Trump's, he said.

Ultimately, no court has determined whether ownership by a president or government official of a corporation that sells products to a foreign government would violate the Constitution, he said, and that could present an additional barrier to success.

(Reporting by Andrew Chung in New York and Dan Levine in San Francisco; Editing by Amy Stevens and Cynthia Osterman)