WASHINGTON — The Senate Republican tax bill, which had appeared to be cruising to victory, suffered a setback late on Thursday as lawmakers were forced to contemplate significant changes, including future tax increases, to help pay for the legislation.
The bill had gained momentum in the morning when it picked up a key swing vote. But it came to a grinding, if temporary, halt, as senators scrambled in the early evening to find ways to raise several hundred billion dollars after some members objected to moving forward without a plan to safeguard against ballooning the deficit.
Lawmakers are now considering alternatives, including reinstating the alternative minimum tax on some corporations and wealthy individuals, and raising the corporate rate above 20 percent after some number of years. A final vote on the bill is expected on Friday after a series of amendments are considered.
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The push to raise money came after an analysis by the congressional Joint Committee on Taxation found that the tax cuts would not pay for themselves by generating enough revenue through economic growth to offset the tax cuts, as Republicans have claimed, but would instead add $1 trillion to budget deficits over the next 10 years.
Senator Bob Corker, Republican of Tennessee, had already been on the fence over deficit concerns, and he insisted on Thursday evening that the Senate leadership find a way to mitigate the cost of the bill, lawmakers said.
“Senator Corker has been pretty clear he doesn’t want any deficit spending,” said Senator John Cornyn of Texas, the No. 2 Senate Republican.
The last-minute attempt to find revenue slowed, at least temporarily, what had appeared to be a cascade of momentum for the bill. Republicans picked up a key swing vote, Senator John McCain of Arizona, earlier in the day, and had appeared to be on track to pass the bill along party lines.
Now, they are under pressure to cut the cost of their bill by as much as one-third, a situation that could require Republicans to insert future tax increases into what was posited as a giant tax cut. That could complicate the final approval of the tax rewrite, particularly with House Republicans, who will be loath to approve a bill that would effectively raise taxes on companies and individuals after a period of lower taxes.
Several senators remain on the fence over the bill, and Republicans can lose no more than two of their members to pass the legislation without any Democratic support.
Mr. Corker, along with the Republican senators Jeff Flake of Arizona and James Lankford of Oklahoma, have expressed concern about piling up more debt as a result of the $1.5 trillion tax overhaul. Other Republican senators, like Ron Johnson of Wisconsin, have objected to how the bill treats businesses whose profits are distributed to their owners and taxed at rates for individuals.
During a procedural vote on Thursday that suddenly turned dramatic, Republican leaders huddled with Mr. Corker, who had wanted to add a triggerlike mechanism to the bill that would force future tax increases if federal revenues fell short of projections. The Senate parliamentarian deemed that trigger out of bounds under the budget rules that Republicans must abide by in order to shield their bill from a Democratic filibuster.
Mr. Corker, Mr. Flake and Mr. Johnson withheld their votes on a Democratic motion that would have relegated the bill back to a Senate panel, before finally relenting and joining their Republican colleagues in defeating the motion. The floor debate on the bill continued, and Republicans were discussing alternative provisions such as slowly raising the corporate rate above 20 percent.
“I’m sorry we hit this bump in the road late, because we were moving so well,” said Senator Johnny Isakson, Republican of Georgia.
Asked if the problem could be fixed, he replied, “Anything can be fixed.”
On Thursday afternoon, Republicans dismissed the joint committee projections that the bill would lead to additional economic growth of 0.8 percent over a decade, well short of the acceleration needed for the tax cuts to pay for themselves over that time. The analysis said the tax cuts would generate about $458 billion in revenue over a decade, but would also require about $51 billion in additional interest costs. That would leave the bill with a $1 trillion price tag.
The joint committee figures “pointed out that there is significant economic growth,” Mr. Cornyn said. “We think they lowballed it.”
Even before the parliamentary snag, critical components of the bill remained under debate, including the size of the corporate tax cut and whether it will retain any ability for individuals to deduct state and local taxes. Still, Republican leaders expressed optimism that they were close to approving the bill.
“We’re on the cusp of a great victory for the country,” Senator Mitch McConnell of Kentucky, the majority leader, said earlier in the day, adding that Senate Republicans were “headed toward the finish line either late tonight or early tomorrow.”
“I’m ready to vote,” said Senator John Kennedy, Republican of Louisiana. “It is time for us to saddle up and ride and go vote.”
Mr. McCain, one of the three Republicans who sank the party’s attempt to repeal and replace the Affordable Care Act earlier this year, released a statement on Thursday saying that he would vote for the tax bill. One of the other health care holdouts, Lisa Murkowski of Alaska, had said on Wednesday that she would vote for the tax bill as well.
“I believe this legislation, though far from perfect, would enhance American competitiveness, boost the economy, and provide long overdue tax relief for middle-class families,” Mr. McCain said.
Mr. McCain was seen as a wild card because of his willingness to buck his party’s leadership in the health care vote. He also voted against big Republican tax cut packages in 2001 and 2003.
But Mr. McCain said that he was satisfied that the tax overhaul had gone through “regular order” in the Senate, with sufficient public hearings and opportunities for amendments. While he said he took seriously concerns that colleagues had raised about the deficit, Mr. McCain said that on balance it would be good for the country.
“It’s clear this bill’s net effect on our economy would be positive,” he said.
Other senators remained undecided on Thursday, including Susan Collins, Republican of Maine, who said she was optimistic that her concerns would be addressed but was not yet ready to support the legislation.
“I am not committed to vote for this bill because who knows what is going to happen on the Senate floor,” she said at a Christian Science Monitor breakfast on Thursday morning.
Ms. Collins said she remained concerned about the impact of the Senate plan to repeal the Affordable Care Act’s mandate that most Americans have insurance or pay a penalty, and she also wants to add a provision allowing individuals to deduct up to $10,000 in property taxes.
On the Senate floor on Thursday morning, Senator Chuck Schumer of New York, the Democratic leader, criticized Republicans for how they had undertaken the tax overhaul, complaining that they had shut out Democrats as they put together their bill.
Mr. Schumer said the Republican tax bill had made “a mockery of the legislative process,” and he pleaded for Republicans to work with Democrats on taxes instead of moving forward with the current tax plan.
“If my Republican friends close the door on their partisan tax bill tonight,” he said, “they will find an open door for bipartisan tax reform tomorrow.”
If the bill clears the Senate, it would need to be reconciled with the House-passed version of the bill, which differs substantially from the Senate version.