By Lindsay Wise and Siobhan Hughes

WASHINGTON -- Republican senators are bracing for major defections on the next coronavirus aid package as GOP lawmakers' antipathy toward government spending runs into the turbulence of election-year politics.

With the country in the grip of a widespread pandemic, Republicans are deeply divided over the scope and scale of aid needed to keep the economy afloat -- and whether their party's precarious standing in polls justifies setting aside ideological concerns about the fast-growing deficit less than 100 days before voters head to the polls.

Sen. Lindsey Graham (R., S.C.) made a stark prediction over the weekend about the consequences of the infighting: "Half the Republicans are going to vote no," he said on Fox News on Sunday. "That's just a fact."

The stimulus debate pits the GOP's political pragmatists against its budget hawks, with the fate of swing-state incumbents hanging in the balance: At-risk Republican senators don't want to return to the campaign trail during the August recess empty-handed, while fiscal conservatives recoil at any plan that they see as ballooning the deficit and conditioning the public to expect broader government assistance once the pandemic is over.

At stake could be control of the Senate and White House, some Republicans warn. The nonpartisan Cook Political Report last week released a new analysis of key Senate races that for the first time this cycle favored Democrats to take back the chamber.

Democrats already control the House and are expected to keep or expand their majority in November, making the GOP-held Senate a critical bulwark against total Democratic control of the legislature next year. Democrats need to flip three seats from red to blue to seize control of the chamber in November, or four if President Trump wins re-election.

Some Republicans are grumbling privately that Senate Majority Leader Mitch McConnell (R., Ky.) and Treasury Secretary Steven Mnuchin waited too long to start putting a bill together and the process hasn't been transparent. They are worried that the GOP's opening $1 trillion bid puts them at a disadvantage before talks with Democrats even start. "If you're starting at $1 trillion it's only going to go up and that's a lot of the concern from the fiscal conservatives," one Senate GOP aide said.

"If the White House thinks that they're going to just be able to count on all 53 Republicans to vote with whatever they say, that's going to be a lot harder than they think," the aide said, adding that it is possible that the Senate might pass a temporary fix for enhanced unemployment insurance, which is set to expire July 31, and push the rest to September.

No Republicans are thrilled about so much more spending, said another GOP aide. "At the same time, this bill is not going away because there are imperatives to doing this bill, there are imperatives to spending...If you lose some of the fiscal conservatives by doing something, how many other people do we lose if we do nothing? These are extraordinary times and we can't beat the virus by doing nothing."

The Senate GOP plan unveiled on Monday would fund another round of direct payments to Americans, $105 billion for schools and universities and additional assistance for small businesses. It also would provide liability protections for schools, businesses and health-care providers, $16 billion more for testing and contact tracing in states, and $26 billion to develop and distribute a vaccine. The proposal extends unemployment benefits, although it would cut the current federal $600 weekly supplement to $200 a week through September.

At a press conference Monday, Mr. McConnell stressed that this was just the beginning of the process. "We can't pass a bill in the Senate without Democrats, nor obviously can it pass the House," he said. "The Republicans are in the majority in the Senate. This is the starting place."

One of the most vocal critics of the estimated $1 trillion stimulus bill drafted by the White House and Mr. McConnell is Sen. Ted Cruz (R., Tex.), who says he is a "no" vote.

"There is significant resistance to yet another trillion dollars," Mr. Cruz told reporters on Monday. "The answer to these challenges will not simply be shoveling cash out of Washington. The answer to these challenges will be getting people back to work."

Mr. Cruz made a similar case behind closed doors last week at a lunch for GOP senators, according to people familiar with his remarks.

Sen. Tom Cotton (R., Ark.) told his Republican colleagues at the same lunch that they should pay attention to what the vulnerable Senate Republicans needed from the package and accommodate them even if it meant more spending. He argued that if Democrats won the Senate and Joe Biden won the presidency, they would pass much more expensive legislation, and that the GOP should take steps now to avoid such an outcome.

Mr. Trump and Treasury Secretary Steven Mnuchin later called Mr. Cotton to thank him for making the argument, according to a Republican familiar with the conversation.

The White House didn't respond to a request for comment.

Sen. Cory Gardner (R., Colo.) told reporters after the lunch that he had advocated for "a lot" of things, from reloaded funds for the small-business Paycheck Protection Program and business tax credits, to help for restaurants and extended unemployment benefits.

"The American people need help," he said. "Let's help them."

But conservative advocacy groups and activists have mobilized to push back hard against Mr. McConnell and Mr. Mnuchin's plan, even before the text was released. Club for Growth, a conservative advocacy group, started telling lawmakers privately on Wednesday that it planned to urge a "no" vote against Mr. McConnell's bill, and would penalize any senators who voted for it in the group's high-profile congressional scorecards. That was a change from the Club's relative silence on previous coronavirus aid packages that passed the Senate in February and March.

David McIntosh, Club president, said Mr. McConnell is encountering resistance within his own party because he and Mr. Mnuchin have "cooked up a trillion dollars in spending and did nothing in the bill to actually stimulate economic growth." Mr. McIntosh bemoaned the lack of payroll tax cuts and parental choice grants, priorities for conservatives.

"I think the president has been badly served by Mnuchin and McConnell when they just decided we're going to run another one of these big spending bills," Mr. McIntosh said. "And we're saying very publicly now that if McConnell has his way, that Republicans will lose, that he will have led the Republicans into the minority in the Senate...Voters will say, 'Why elect Republicans when we can get Democrats to give us handouts? We want Republicans to rebuild the economy.'"

Asked on Monday whether Republicans' disagreements will weaken Mr. McConnell's hand as he enters into negotiations with Democrats, Sen. Roy Blunt (R., Mo.) said no.

"When it comes to spending money on these kinds of things, a number of Republicans are just not going to be willing to do that," acknowledged Mr. Blunt, a member of Mr. McConnell's leadership team. But he dismissed as unlikely the prospect that Republicans might be forced to pass the coronavirus legislation without support from a majority of their own members.

Mr. Blunt said he hopes that the needs of the party's front-line lawmakers in tough races are at the forefront of his colleagues' minds.

"My guess is that they all want to see a result," he said, "and we need to produce a result."

--Andrew Duehren contributed to this article.