After months of slowly building support among Democrats in Congress for official impeachment proceedings against President Donald Trump, the latest allegations involving the President reportedly discussing an investigation of a top Democratic candidate with the leader of Ukraine are pushing more calls for the House to undertake a formal impeachment probe.

"This ticks all the boxes," said Rep. Mary Gay Scanlon (D-PA), one of many newly-elected Democrats who argue that President Trump's actions can not be ignored.

"The president of the United States may have used his position to pressure a foreign country into investigating a political opponent," a group of freshmen Democrats argued in a joint statement issued Monday night, as they denounced the refusal of the Trump Administration to release information from a whistleblower in the U.S. Intelligence Community.

"This flagrant disregard for the law cannot stand," wrote Gil Cisneros (D-CA), Jason Crow (D-CO), Chrissy Houlahan (D-PA), Elaine Luria (D-VA), Mikie Sherrill (D-NJ), Elissa Slotkin (D-MI), and Abigail Spanberger (D-VA), all of them with military backgrounds - and most from very competitive districts.

"If these allegations are true, we believe these actions represent an impeachable offense. We do not arrive at this conclusion lightly," the group said, joining in a call for the White House to release the transcripts of Mr. Trump's phone call, and any other documents related to the call to investigate former Vice President Joe Biden, and his son Hunter, over his work in Ukraine.

Not only was support for an impeachment investigation building over the Ukraine episode, but also simply because of the refusal of the White House to turn over documents, and allow witnesses to testify on Capitol Hill in a variety of investigations.

One by one, Democrats had been declaring their support for a more intense investigation - but it was clear the situation involving the whistleblower complaint, which was found to be both 'credible' and 'urgent,' with the White House refusing to give the information to Congress, as required by federal law.

"This is a defining moment," said Rep. John Larson (D-CT), a senior Democrat who had resisted calls for impeachment, as he said if the acting Director of National Intelligence refuses to give information from the whistleblower to Congress - that will be the last straw.

"If the Director refuses to comply at Thursday’s hearing, the Trump Administration has left Congress with no alternative but for the House to begin impeachment proceedings, which I will support," Larson said.

"This is a matter of grave urgency," said Rep. Rosa DeLauro, another veteran Democrat from Connecticut, who accused the President's lieutenants of 'stonewalling' the Congress on this whistleblower matter.

"The horse is out of the barn," said Geoff Garin, a veteran Democratic pollster and political strategist. "Saddle up."

After months of side stepping by House Speaker Nancy Pelosi, there was a sense on Monday that something dramatic had changed - all because of the revelations involving the Trump-Ukraine call.

Congress is set to leave town later this week for a two week break - and not return until mid-October.

It was not clear if Democrats would wait to vote to authorize a full impeachment investigation until next month.