On Tuesday, the top US diplomat in Ukraine, Bill Taylor, told House lawmakers that the Trump administration made military aid to Ukraine conditional "for domestic political reasons."

The White House has maintained that the president did not attempt a quid pro quo arrangement with Ukraine involving the aid — the focus of House Democrats' impeachment inquiry.

Taylor's testimony most likely reduces the options for Republicans seeking to defend of the president from impeachment.

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The closed-door congressional testimony Tuesday of the top US diplomat in Ukraine could be the development that forces Republicans to reassess their defense of President Donald Trump from a swirling impeachment process.

On Tuesday, Bill Taylor told the House Intelligence Committee in unambiguous terms that the White House — and its emissaries he described as conducting a shadow foreign policy — had made military aid to Ukraine conditional on the Ukrainian president announcing investigations into the 2020 US Democratic presidential candidate Joe Biden and his son Hunter and into a conspiracy theory involving the 2016 US election.

Taylor described in detailed terms a quid pro quo arrangement that Trump and Republicans have denied ever took place. The Latin phrase describes a favor granted explicitly in return for something else.

(Though the testimony was private, outlets including CNN, The New York Times, and The Washington Post obtained his full opening statement.)

He said that Gordon Sondland, the hotel magnate and GOP donor who Trump appointed as ambassador to the European Union, told him that "everything" including security assistance was dependent on Ukraine announcing the investigations.

"I became increasingly concerned that our relationship relationship with Ukraine was being fundamentally undermined by an irregular, informal channel of US policymaking and by the withholding of vital security assistance for domestic political reasons," he told lawmakers.

Up to that point, even top Republicans who acknowledged that Trump had pressured Ukrainian President Volodymyr Zelensky in a July 25 phone call to pursue the investigations mostly stuck to the line that no compelling evidence of an explicit quid pro quo had emerged — and that there were therefore no grounds for impeaching the president.

As recently as Saturday, Sen. Lindsey Graham, one of the president's key congressional allies, told Axios that he had not seen any evidence so far to prove the claim.

Sen. Lindsey Graham of South Carolina. Jonathan Ernst/Reuters

"If you could show me that, you know, Trump actually was engaging in a quid pro quo, outside the phone call, that would be very disturbing," he said.

The White House itself briefly appeared to acknowledge a quid pro quo.

In a stunning press conference last week, the acting White House chief of staff, Mick Mulvaney, described and defended a quid pro quo deal involving military aid. He hastily retracted the comments, however.

Taylor's testimony has made it even more difficult for Republicans to maintain the "no quid pro quo" line of defense and has strengthened the hand of Democrats.

Democratic Rep. Debbie Wasserman Schultz told The Washington Post it was "the most damning testimony I've heard."

So what next for Republicans seeking to win the battle for public opinion as the House Democrats' case for impeaching Trump grows stronger?

According to multiple reports, the White House has not formed a "war room" to coordinate the party's response to the inquiry, leaving efforts to hit back at Democrats conducting the investigation in disarray even before Taylor's testimony.

The situation was such that, according to Politico, Steve Bannon — the White House chief strategist who left on acrimonious terms — was launching a podcast that he wants to act as an informal war room for the Republican defense against impeachment.

President Donald Trump at a Cabinet meeting at the White House on Monday. Associated Press

The Post correspondent Dan Balz spelled out some of the routes that Republicans might take after Taylor's testimony.

"Will they conclude that what the president did was legitimate?" he wrote. "Will they attempt to point in other directions? Will they argue that what Trump did wasn't right but isn't impeachable? There's less room for equivocation about what happened today than there was before."

The White House's immediate response was to attack Taylor's credibility, as seen when the press secretary, Stephanie Grisham, accused him of partisan bias, echoing White House attacks on the officials who conducted the special counsel's Russia investigation.

"This is a coordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the Constitution," Grisham said.

Trump in a tweet early Wednesday quoted GOP Rep. John Ratcliffe, who in an appearance on "Fox and Friends" argued that because Ukraine was not aware that military aid had been withheld during most of the Trump administration's negotiations, a quid pro quo never existed. But in fact, Taylor did say in his testimony Tuesday that Sondland told Ukraine's president that if he did not publicly announce the investigations, US-Ukrainian relations would be at "stalemate," in what Taylor understood to be a reference to military aid being frozen.

And in what is likely to cause further concern for the White House, a Reuters/Ipsos poll — taken before Taylor's testimony — showed support among independent voters for impeaching Trump soaring.

But there was also a clue in the poll showing cause for optimism for Trump.

Despite the official White House line of defense to impeachment being in tatters, Trump still has good reason to believe that Republicans will find a new argument in favor of acquitting him in a prospective Senate impeachment trial.

The survey found that almost three-quarters of Republicans approved of Trump's handling of foreign policy.

This was down from a previous survey, but the high levels of support from the party's grassroots will most likely deter Republicans considering abandoning their defense of the president.