The alarm bells the senator sounded came amid growing worries among Republicans about the party’s lack of legislative achievements. (Manuel Balce Ceneta/AP)

Wealthy conservative donors and influential Republican lawmakers say they increasingly fear a historic backlash at the ballot box next year if the GOP effort to pass a sweeping rewrite of the nation's tax laws falls short in the coming months.

At a two-day midtown Manhattan summit of the billionaire industrialist Koch brothers' powerful donor network, GOP patrons, senators and strategists spoke in cataclysmic terms about the price they expect to pay in the midterm elections if their tax reform effort does not win passage.

They voiced concerns a demoralized Republican base would stay home, financiers would stop writing campaign donation checks to incumbents and the congressional majorities the party has built in the House and Senate could evaporate overnight.

To head that off, the same Republicans said they are waging an intense, multi-front effort in and outside of Congress and the White House to shepherd the endeavor to the finish line.

Koch network officials said they have invested more than $10 million this year in advocating for the GOP tax plan.

They are sensitive to lingering intraparty divisions and distractions from President Trump that could sink their attempt. Those complications heightened tensions inside the luxe hotel ballrooms where the summit wrapped up Friday.

"Hugely nervous," said Chris Wright, an oil and gas executive from Colorado, describing the tone of the conversations he and other donors were having about tax reform. Wright, like most attendees, argued that passing a tax bill would give the party a much-needed boost.

And if they fail?

"I think the Republicans will pay a heavy price in the midterm elections," he said.

Art Pope, a major conservative donor from North Carolina, put it this way: "When you have lack of success, that may depress voter turnout for Republicans, that may depress donations for Republicans and conservatives."

Sen. Ted Cruz (R-Tex.) warned that Republicans could face a "Watergate-level blowout" in the midterm elections if they don't make major legislative strides on taxes and health care, invoking the political scandal that brought down Richard Nixon's presidency and set back the GOP considerably in subsequent elections.

"If tax reform crashes and burns, if [on] Obamacare, nothing happens, we could face a bloodbath," said Cruz, who spoke in a moderated discussion.

More than 100 top donors gathered for the Koch network meeting. The Washington Post and other news organizations were invited to cover the gathering, on the condition the donors present not be named without their permission.

In a panel discussion focused on tax reform, Sens. David Perdue (R-Ga.) and Tim Scott (R-S.C.) warned of dire political consequences if the endeavor is not successful.

"Failure is a starting process, in my opinion, to losing the House, which will manifest in 2018 if we don't get this done," Scott said. "And frankly, I think it destabilizes the Senate, we lose the Senate as well."

Trump and Republican congressional leaders are aiming to turn their plan into law in the next three months. An initial proposal crafted by the White House and congressional GOP leaders aims to cut tax rates for the wealthy, the middle class and businesses, as well preserve some popular deductions.

"I mostly came here today to assure you, that before this year is out, with your help in this room, we're going to cut taxes across the board," Vice President Pence said in a speech to the Koch donors Friday afternoon.

Republican leaders are hopeful if they accomplish that, some of the disappointment in the party over the failure to dismantle the Affordable Care Act this year will evaporate. But the GOP discord that decimated that effort is already threatening to undermine the tax push.

"I think we can get there, but I'm very, very concerned about it right now," Perdue said. Asked whether individual senators understand the importance of uniting broadly and avoiding fierce fights for individual demands in the tax talks, he replied: "I think 48 United States senators get that."

The crowd, well aware there are 52 Republican senators, laughed.

The Koch network plans to spend between $300 million to $400 million on policy and political campaigns during the 2018 election cycle. Within that budget, they have already invested eight figures on the tax push.

They are airing television ads, hosting events and rallying donors to write checks, call lawmakers and pen op-eds in an effort to ramp up pressure.

"It's the most significant federal effort we've ever undertaken," said Tim Phillips, the president of Americans for Prosperity, a national Koch-backed organization.

They are hoping Trump complements their efforts. As he has throughout his presidency, Trump has been consumed in recent weeks by other issues and fights. One such feud is with Sen. Bob Corker (R-Tenn.), a retiring lawmaker who is seen as a key figure when it comes to tax and budgetary matters.

"It's frustrating and annoying that he does that," said Wright's wife, Liz Wright, speaking of the president's tendency to stoke conflict on social media. She sought to play down the impact of the president's various fights on the tax effort.

Trump has also reopened a fierce debate over health care with executive actions this week taking aim at the ACA. Those moves could steal away attention from taxes and increase the potential for a government shutdown in December.

There are also brewing tensions between Senate Republican leaders and the rank and file in the wake of the failed repeal push. Those could also add thorns to the tax talks.

In a dig at his own caucus's dearth of achievements, Cruz called the Senate's confirmation of Neil M. Gorsuch to the Supreme Court "damn near the only thing Senate Republicans have done." Cruz also argued the Senate leaders should keep the upper chamber in session for more days each week.

"It's the old bulls who don't want to stay in session that long," Cruz said.

Amid that internal rancor, Republican lawmakers are dealing with external pressure to finish their work on tax reform that is as high as it has been for a legislative priority this year.

Failure, to many of the people who helped elect them, would fundamentally transform their standing in a way the health-care stumble did not.

"They risk their activist base becoming disillusioned. I think a lot of the donor base would also be disillusioned. So there's a great deal at stake for them," Phillips said.

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