WASHINGTON – While Democrats stepped up calls for impeachment, President Donald Trump directly acknowledged Sunday that he spoke with the president of Ukraine about an investigation into political opponent Joe Biden, but called the talk appropriate.

Speaking with reporters en route to a trip to Texas and Ohio, Trump said: "The conversation I had was largely congratulatory, was largely corruption – all of the corruption taking place – it was largely the fact that we don't want our people like Vice President Biden and his son" contributing to the corruption already in the Ukraine.

In statements and tweets in recent days, Trump has implied that Biden was a subject of the call with the Ukraine leader; Sunday was his first direct acknowledgement, as pointed out by a number of lawmakers.

"FYI - if Trump chooses to admit the corruption, it doesn’t all of a sudden make it not corrupt," tweeted Sen. Chris Murphy, D-Conn.

Members of Congress called for more details on a complaint by a whistleblower who said that, in a July phone call, Trump pressured Ukraine President Volodymyr Zelensky to have his government investigate Biden and his son.

Biden, whose son Hunter had business interests in Ukraine, said Trump is using his job as president to get a foreign government to dig for dirt on a political rival.

“He’s doing this because he knows I’ll beat him like a drum," Biden told reporters on Saturday. "And he’s using an abuse of power and every element of the presidency to try to smear me."

Democratic lawmakers said the incident will likely increase calls for impeachment of Trump, who is being probed for his actions regarding Russian interference in the 2016 election.

"I have spoken with a number of my colleagues over the last week, and this seems different in kind," said Rep. Adam Schiff, D-Calif., chairman of the House Intelligence Committee, speaking on CNN's State of the Union.

"And we may very well have crossed the Rubicon here," he said.

The Ukraine-related allegations against Trump are included in the whisteblower's complaint that the office of Director of National Intelligence is refusing to release to Congress.

In demanding that the administration release the document, Democrats cited reports that Trump made aid to Ukraine contingent on its willingness to investigate Biden.

House Speaker Nancy Pelosi, D-Calif., who has expressed skepticism about impeachment of Trump, indicated she could change her mind if the administration continues to refuse to provide the complaint. "They will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation," Pelosi said in a letter to House colleagues.

At least one Republican also called for more information. Saying it is "critical for the facts to come out," Sen. Mitt Romney, R-Utah, tweeted: "If the President asked or pressured Ukraine’s president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme."

At the White House, Trump said he would have "no problem" if his attorney Rudy Giuliani testified before Congress about Ukraine and Biden.

Giuliani, who has spoken with Ukraine officials about Biden, has declined to discuss his conversations with Trump on the matter, citing attorney-client privilege. In an appearance on Fox News Sunday, Giuliani said an investigation is warranted over the business interests of Hunter Biden, the former vice president's son.

Trump echoed his defense of the Ukraine call during a stop in Houston, and said his team may put out a transcript in some form. "We’ll make a determination about how to release it," he said.

Trump spoke with President Zelensky by phone on July 25.

U.S. Secretary of State Mike Pompeo spoke with Ukraine’s foreign minister, Vadym Prystaiko, on Sept. 17, about five days after Trump released $250 million in American military aid to the country.

According to a State Department readout of the call, Pompeo “affirmed U.S. support for Ukraine as it advances critical reforms to tackle corruption, strengthen the rule of law, and foster an economic environment that promotes competition and investment."

On Saturday, Prystaiko told a Ukrainian news outlet that Trump did not pressure the country’s president. “I know what the conversation was about, and I think there was no pressure,” he told Hromadske.

The Ukraine dispute revolves in part around Biden's work for President Barack Obama.

Acting in his role as vice president, Biden urged Ukraine's then-government to fire its chief prosecutor, tying the decision to U.S. aid. Other world leaders endorsed the move, describing the prosecutor's office as hopelessly corrupt.

Trump, Giuliani, and other critics said the former prosecutor was investigating a company of which Hunter Biden was a board member.

Sen. Ben Cardin, D-Md., speaking on Fox News Sunday, said Biden's actions were known and supported at the time, and there is no evidence of impropriety. Cardin said Trump's accusations against the Biden family are an attempt to distract voters from his own problems.

Biden has said he has never spoken with his son about the latter's business dealings in Ukraine.

Trump challenged that statement, telling reporters: "Give me a break."

House Democrats are already investigating Trump on claims he tried to obstruct justice during special counsel Robert Mueller's investigation of Russian attempts to influence the 2016 election.

Trump has denied collusion with Russians in 2016 or obstruction of justice in the years thereafter. He has described the various investigations as "witch hunts,' and said that now includes the whistleblower's claims about his Ukraine call.

"They keep wasting time in Congress over nothing," Trump said Sunday. "They are studying nothing."

Trump spoke during travel to Texas and Ohio in the company of foreign leaders.

First came a visit to Houston for an event with India Prime Minister Narendra Modi at the city's pro football stadium.

Trump then traveled to Wapakoneta, Ohio, to visit a new Australian-owned manufacturing plant, along with that nation's prime minister, Scott Morrison.

Trump then heads to New York, where he will attend United Nations meetings over the next three days – including a meeting with Zelensky, the Ukraine president.

Contributing: Deirdre Shesgreen