Pelosi made her comments at a CNN town hall Monday evening moderated by anchor Chris Cuomo in the wake of a Washington Post report citing current and former US officials saying Trump revealed highly classified information in a meeting with Russian Foreign Minister Sergey Lavrov and Ambassador Sergey Kislyak last week.
"It undermines the trust that we would have with our allies," Pelosi said.
National Security Adviser H.R. McMaster pushed back on the report from the White House Monday evening, saying, "I was in the room. It didn't happen."
Pelosi said McMaster's response was not satisfactory.
"I thought that the statement of McMaster's was sad for him, for him to be able to come out and speak in that way," Pelosi said.
She also knocked Trump for his "messy" approach to secret government information.
"We cannot have the President of the United States being casually loose-lipped about confirming something, even if it's in the public domain, to an adversarial nation," Pelosi said. "This is dangerous."
Pelosi said the news of Trump sharing classified information with Russian officials was just the latest story that made her demand more information about any connections between the President and Russia.
"As Yogi Berra said, 'It's too much of a coincidence to be a coincidence,'" Pelosi said. "You have to be careful because this is its own incident, but it's about Russia."
She said there needed to be an independent investigation to look into questions of financial, political and personal connections between Trump and Russia.
"Every day I ask the question: What do the Russians have on Donald Trump?" Pelosi said. "He's always catering to them."
The US intelligence community said Russia attempted to influence the 2016 election to hurt Hillary Clinton and boost Trump's candidacy. Former FBI Director James Comey, who Trump fired last week, said in March that the bureau was investigating potential collusion between the Trump campaign and Russia.
Trump and those associated with his campaign have denied any collusion.
Pelosi also warned her fellow Democrats against promoting hearsay on this matter, and said Democrats discussing impeachment were doing so without any case to make.
"I don't subscribe to that," Pelosi said of impeachment. "What are the facts? ... If you don't have that case, you're just participating in more hearsay."
A 26-year-old member of the audience, Kati McFarland, said she was diagnosed with a incurable genetic disorder called Ehlers-Danlos syndrome and could die if the protections of the Affordable Care Act, also known as Obamacare, were repealed and replaced with the American Health Care Act, the bill Trump endorsed and the House passed earlier this month.
McFarland asked Pelosi what she would do to protect health care access for those with pre-existing conditions and others who had received health care under Obamacare.
Pelosi lambasted the GOP health care bill in response.
"The whole point right now is to defeat that bill," Pelosi said. "This is a death panel bill because people will die."
The Congressional Budget Office said it will have
an assessment later this month of the version of the AHCA passed by the House.
Its assessment of an earlier version
of the bill said that by 2026, 52 million people would be uninsured. The CBO said 28 million would be uninsured by 2026 if Obamacare remained the law of the land.
Pelosi said Obamacare had "good bones," meaning structurally the law was sound, though she admitted it could still be improved.
War on drugs
Norman Brown, who served 20 years in prison for drug crimes before former President Barack Obama commuted his sentence, asked Pelosi what she would do to stop Attorney General Jeff Sessions from rolling back Obama-era reforms to the Department of Justice.
Pelosi cited previous bills to reduce disparities between crack and powder cocaine sentences, which critics said unfairly targeted people of color. She said those kind of past experiences indicated bipartisan opposition to hard-line sentencing for non-violent drug crimes.
"I think that we will work together in a bipartisan way," Pelosi said.
Last week, Sessions issued a memorandum directing federal prosecutors to seek the toughest possible sentences in drug cases.
"What the attorney general did was irresponsible," Pelosi said.