Democrats in the House and Senate were left frustrated Wednesday after President Trump openly questioned the constitutionality of a new sanctions bill that he signed into law.

Trump signed the bill imposing sanctions on Iran, Russia, and North Korea, but complained that it's "flawed" and that portions of it that limit his ability to conduct foreign affairs the way he wants likely violate the Constitution.

Trump said he signed the bill for the sake of "unity" and said he would honor a section of the law allowing Congress to review his decisions to waive sanctions against Russia. But that language was alarming to Democrats, who said they would be closely watching to see how Trump adheres to the law.

"Even as the president signed the new sanctions bill rather than face an almost certain veto override, his statements cast doubt on his commitment to enforcing these new measures," said Rep. Eliot Engel, D-N.Y., the top Democrat on the House Foreign Affairs Committee.

"We passed this legislation along with a clear message for the White House: if you don't hold Russia accountable, we will," Engel said. "I hope Congress doesn't need to invoke the review provisions written into this law, but if the president continues to cozy up to Russia, lawmakers won't hesitate to act."

Trump also reacted by saying he can make "far better deals" on his own, without legislation from Congress, which drew a rebuke from Sen. Ben Cardin, D-Md.

"This is not about President Trump and his negotiation skills, it's about America," said Cardin, the top Democrat on the Senate Foreign Relations Committee. "Each of these countries present serious challenges for the United States and the new law gives our country a stronger hand in dealing with Russia, as well as Iran and North Korea."

Cardin also said he expects Trump to apply real pressure on Russia. Democrats have been worried that Trump will go easy on Russia because of their perception that he has close ties to Russian President Vladimir Putin, which is why they insisted on language letting Congress review decisions to waive sanctions.

"I expect the administration to deploy diplomats and Treasury officials in the coming days to work closely with our allies around the world and lead a sustained effort to enhance pressure on Moscow," Cardin said. "I remain very concerned that this administration will seek to strike a deal with Moscow that is not in the national security interests of the United States."