"Pursuing the leader of another nation to investigate a political opponent to help win an election is not the conduct of an American president," Biden said. "The allegations that blocked hundreds of millions of dollars, that he blocked hundreds of millions of dollars in congressionally approved money to another country is an allegation, unless he agreed to smear a political opponent is not the conduct of an American president."

Biden, who did not take questions after his brief remarks, did not specifically reference that he and his son, Hunter, were at the center of the burgeoning controversy facing Trump. Trump is accused of pressuring Ukraine's president into launching an investigation into the Biden family.

"I can take the political attacks," Biden said. "They'll come and they'll go, and in time they'll soon be forgotten, but if we allow a president to get away with shredding the United States Constitution, that will last forever.

The remarks are not as forceful as some of Biden’s Democratic competitors in the 2020 race, his comments mark a shift by the former vice president, who said on Saturday said he was not yet ready to call for impeachment.

While addressing reporters in Iowa on Saturday, Biden accused Trump of an “overwhelming abuse of power” whose alleged actions on a telephone call with the president of Ukraine “crosses the line.”

Still, Biden stopped short of calling for impeachment.

“He could be impeached, but I’m not making that judgment now,” Biden said when asked directly if Trump should be impeached. “The House should investigate this. This appears to be an overwhelming abuse of power to get on the phone with a foreign leader who is looking for help from the United States and ask about me and imply things if that's what happened.”

Biden’s reticence on impeachment reflects his pragmatic approach, given that the Senate is unlikely to convict, said former Pennsylvania Gov. Ed Rendell, a Biden campaign surrogate.

“He understands politics,” Rendell said of Biden’s cautious approach to impeachment. “The nation is going to be agitated, Trump supporters will be furious. It will have accomplished nothing and we’ll have stoked up his base. I don’t know if we can match that with record turnouts.”

But, Rendell said, recent revelations on Ukraine may have pushed Biden’s evolution on the matter.

“This current thing is so blatantly against the law. It doesn’t really matter if there’s a quid pro quo. You’re breaking the law, that’s a crime in and of itself,” he said. “I think it’s clear cut grounds for impeachment.”

