WASHINGTON -- The former special envoy to Ukraine testified Tuesday he should have realized -- as many of his colleagues did -- that U.S. President Donald Trump was holding up military aid to Ukraine to pressure the country to investigate political rival Joe Biden.

In public testimony before the House impeachment inquiry, Kurt Volker said he understands now, thanks to hindsight and the testimony of other witnesses, that Trump was using the aid to pressure Ukraine to investigate Biden's son, Hunter, and his role on the board of a Ukrainian gas company, Burisma.

But Volker insisted he did not know of the push at the time, despite his deep involvement with Ukrainian officials on a statement -- never released -- that would have committed the country to investigating Burisma and the 2016 U.S. election. Nor did he make the connection after Rudy Giuliani, Trump's personal lawyer, mentioned the allegations against Joe Biden during a July 19 breakfast, Volker said.

"In retrospect I should have seen that connection differently, and had I done so, I would have raised my own objections," Volker said Tuesday in his opening statement.

Volker was testifying alongside former White House national security official Tim Morrison in the second hearing of the day in the House's impeachment inquiry, the fourth in history against a U.S. president. Both witnesses were requested by Republicans.

Morrison told investigators he was only testifying to present facts -- not to speculate on the identity of the Ukraine whistleblower, nor to disparage his colleagues.

"I am not here today to question their character or integrity," he said, as the White House tweeted out a previous Morrison quote that questioned the judgment of another witness critical of the call, Lt. Col. Alexander Vindman.

Morrison, who stepped down from the National Security Council shortly before he appeared behind closed doors last month, has said he is not concerned that anything illegal was discussed on Trump's July 25th call, something Republicans have repeatedly highlighted.

"As I stated during my deposition, I feared at the time of the call on July 25th how its disclosure would play in Washington's political climate," he said Tuesday. "My fears have been realized."

Democrats say that there may be grounds for impeachment in Trump's push for Ukraine's new leadership to investigate Biden and the 2016 U.S. election as he withheld critical U.S. military assistance.

Trump says he did no such thing and the Democrats just want him gone. He dismissed the hearings as a "kangaroo court."

Republicans have largely stood by his side.

"Welcome back to Act 2 of today's circus, ladies and gentlemen," said Rep. Devin Nunes of California, the ranking Republican on the House intelligence committee, who has lambasted the impeachment inquiry. "It's obvious Democrats are trying to topple the president solely because they despise him."

Associated Press Writers Mary Clare Jalonick, Matthew Daly, Alan Fram, Lisa Mascaro and Laurie Kellman contributed to this report.