Republicans are finding it’s one thing to issue a blanket promise to cut spending, an entirely different matter when you actually take the scissors to $1 of every $6 spent by agencies like the IRS, the FBI, NASA and the National Park Service. Federal layoffs would be unavoidable, the White House warns.

That’s the real-world impact of House Republicans’ campaign promise to cut $100 billion from the budgets of domestic agencies. Next week, they plan to vote on a resolution setting appropriations for the rest of the year at 2008 pre-recession levels in place before President Barack Obama took office.

The vote will be largely symbolic. The actual cuts would have to be made in appropriations bills that would have to clear a 60-vote hurdle in the Senate, where Republicans hold only 47 seats.

The $100 billion promise, contained in the GOP’s “Pledge to America” campaign manifesto, is based on cuts from Obama’s budget recommendations for 2011, but the actual savings in returning to Bush-era levels would be a little less since the government is operating at last year’s slightly lower budget.

Still, compared with 2010 rates and assuming a full year of implementation, Republicans are promising to cut up to $84 billion from nine appropriations bills, cuts that would average 18 percent. Some Republicans, especially in the Senate, may join Democrats in balking when they see their size.

But the House insurgents — backed up by last year’s election mandate — promise they won’t be weak-kneed in fulfilling their drive to shrink the government and wrestle the spiraling national debt under control.

A return to 2008 levels would mean significant cuts for lots of programs favored by Republicans, including an 8 percent cut to NASA, a 16 percent cut for the FBI and a 13 percent cut in the operating budget of the national parks.

There are other political land mines.

Newly elected Republicans in Minnesota, Wisconsin and the Dakotas are sure to feel major political pressure over big cuts looming for the Low-Income Home Energy Assistance Program, known as LIHEAP, which provides home heating subsidies to the poor. Former Appropriations Committee Chairman David Obey, D-Wis., who retired last year, orchestrated a doubling of the program’s budget — from $2.5 billion to $5 billion — since 2008.

Obey said it’s going to mean “a lot of people who aren’t able to pay their heating bills are going to have no way to heat their homes — or they’re going to have to decide to eat less or see the doctor less.”

Republicans in Texas, Florida and Alabama — where NASA facilities mean thousands of jobs — are sure to fight against cuts to the space agency. NASA could have to abandon the International Space Station because of the cuts, the White House warns.

Lawmakers in both parties from rural districts are likely to resist what could be an almost 20 percent cut to a program that subsidizes service by smaller airlines to isolated cities and towns like Scottsbluff, Neb., and Burlington, Iowa. Smaller subsidies or tighter rules would probably mean some communities would lose service.

As local school districts cope with budget squeezes, they won’t be able to count on the same amount of help from the federal government. Special education grants to states could be cut by $1.4 billion, or 11 percent, forcing hometown school boards to cut services or make up the difference with local funds.

The Women, Infants and Children program, which provides food for low-income pregnant women, mothers and young children, has near-universal support. But without an exemption from the cuts, 1 million of them could lose benefits next year, according to calculations by the Center on Budget and Policy Priorities, a liberal research and advocacy group.

“Cutting the big programs isn’t that hard. It’s the little stuff that everybody fights the hardest about, whether it’s LIHEAP or WIC or all this other stuff,” said Jim Dyer, a former top aide for the House Appropriations Committee. “You’re looking at tremendously popular programs like state water grants, the national parks, cancer research, higher education, food safety — all of this stuff’s got to be on the table.”

“We’re not talking about cutting; we’re talking about getting to ’08 levels, which provides a high level of funding for (health research), Pell Grants and for food and drug safety,” said Rep. David Dreier, R-Calif. “To somehow say that we are gutting this is a gross mischaracterization of where we are.”

The account to pay employee salaries at the Food and Drug Administration would actually bear a 35 percent cut — more than $1 billion — if it were returned to 2008 levels.

Pell Grants for college students from low-income families could be cut by more than $1,000 from the current $5,550 maximum grants. A cutback in housing subsidies would mean that hundreds of thousands of people won’t see their Section 8 vouchers renewed. And a $1 billion, 24 percent cut to the historically underfunded Indian Health Service would reduce critically needed health care in some of the most impoverished places in the country.

Republicans aren’t saying that every account will absorb an 18 percent cut back to 2008 levels. The most popular programs might be cut less; others slashed even more.

Republicans have already imposed a 5 percent cut on lawmakers’ budgets for running their offices on Capitol Hill.

“I hope that federal agencies across the spectrum will follow suit and find ways to cut their own budgets,” said House Majority Leader Eric Cantor, R-Va. “If not, we’re happy to do it for them.”

The Pentagon, the Homeland Security Department and veterans’ programs were exempted from the cuts when Republicans drew up the promise, but are likely to get a good scrubbing anyway.

“There are no sacred cows,” said House Appropriations Committee Chairman Harold Rogers, R-Ky.

Cuts to some easy targets, like the National Endowment for the Arts — marked by Republicans when they took over Congress in 1995 — would save relatively little. Its current budget is $168 million; a return to 2008 levels would save just $13 million.

The White House says the promised GOP cuts would fall disproportionately on domestic agencies whose discretionary budgets are passed by Congress each year. They account for only about $1 in $7 spent by the government. Rising Medicare and Medicaid costs are the real drivers of the United States’ long-term deficit woes.

“In terms of the bottom line, we totally agree that there needs to be discipline in discretionary spending, but we shouldn’t for a moment believe that these levels of savings will in and of itself solve the fiscal challenge,” said White House Budget Director Jacob Lew. “The problem is much bigger than the total of nondefense discretionary spending, much less a reduction of it.”