After oral arguments before a federal judge here Thursday, Sen. Richard Blumenthal, D-Conn., said he “couldn’t be more confident” that he would win a legal challenge to President Donald Trump’s deals with foreign leaders.

Blumenthal and other Democratic lawmakers are suing Trump for accepting gifts from foreign leaders before getting approval from Congress, which the senator argued is a violation of the Constitution’s Emoluments clause.

The clause gives Congress control over the president’s acceptance of gifts or favors from foreign leaders.

Blumenthal and his colleagues are taking aim at the Trump International Hotel in downtown Washington, which they say has become a favored watering hole and bunkhouse for foreign official delegations seeking to curry favor with the president.

Although day-to-day operation of the Trump Organization has fallen to Trump’s sons Donald Jr. and Eric while Trump is president, Trump Sr. remains able to draw money out of his international hotel, resort and real estate empire.

“These repeated violations of law have been done because the president has failed to come to Congress for consent,” Blumenthal said. “We’re the only game in town — we’re the only ones who can enforce that right.”

If Blumenthal wins the lawsuit in lower federal courts, it could move forward and become a landmark Supreme Court case that tests the full power of the clause. Such a ruling could clarify the extent to which a president can serve as the leader of the United States while maintaining links to preexisting business interests.

Blumenthal and Rep. Jerrold Nadler, D-N.Y., are plaintiffs in the case along with more than 200 other lawmakers, including Sen. Chris Murphy, D-Conn., and Rep. Rosa DeLauro, D-Conn.

Elizabeth Wydra and Brianne Gorod, the president and chief counsel of the Constitutional Accountability Centers Attorneys, argued on behalf of the lawmakers at Thursday’s hearing.

Blumenthal said Trump defies the clause by making deals with foreign leaders and leaving Congress in the dark.

He said lawmakers are unsure of the extent of the deals because Trump has never brought them before Congress for approval. But, he noted, news stories have uncovered foreign government involvement in what amounts to uncontrolled gifts to the president.

“We cannot vote on what we don’t know,” Blumenthal said. “We know the tip of the iceberg because of the press, but we can’t do our job unless the court moves this forward.”

Defense attorneys said the lawsuit is more of a political attack than a legitimate Constitutional issue.

The presiding judge, Emmett Sullivan, an appointee of President Bill Clinton, said Blumenthal’s case would be stronger with more Republican support.

But Blumenthal maintained that if Trump won’t bring his emoluments before Congress for a vote, then lawmakers can’t avail themselves of their rights under the clause — Democrat or Republican.

Nadler, the New York representative, said Congress can’t gauge whether a gift is taken by the president in the interest of the United States or for private benefit.

He cited a deal between Trump and the Chinese technology firm ZTE which violated American sanctions for its business deals with North Korea and Iran. Trump lifted penalties after a direct plea from the Chinese government, which raised eyebrows amid trade negotiations with China.

“That is the chief anti-corruption provision of the Constitution,” Nadler said of the Emoluments Clause. “Congress must enforce it.”

The case is unique because it argues Trump is allegedly violating lawmakers’ rights, unlike two other lawsuits that question Trump’s improper payments with foreign leaders.

A federal judge ruled in March that Washington, D.C., and Maryland may sue Trump, alleging that the president violated the Constitution’s ban on receiving payments from foreign governments. A judge in New York dismissed a similar pair of cases, which claimed that Trump violated the clause by not fully divesting himself of his real-estate holdings while in public office.