As the House prepares to vote this week on a new five-year farm bill, advocacy groups from across the political spectrum are lining up to oppose it, threatening to derail legislation that died last year when leaders refused to bring it to the floor.

The groups include a litany of conservative and libertarian mainstays like Americans for Prosperity, the American Enterprise Institute, the Cato Institute and the National Taxpayers Union. Some liberal groups oppose the bill in its current form, including the Environmental Working Group, the National Black Farmers Association, the United States Public Interest Research Group and Defenders of Wildlife.

The House bill would save nearly $40 billion in agriculture spending over the next 10 years, with half of the cuts coming from a food stamp program. But the groups say the 1,000-page bill amounts to a nearly $1 trillion giveaway to wealthy corporate farmers, with price supports for crops that distort the market and insurance subsidies that all but guarantee farmers’ income.

Americans for Prosperity said it had begun a grass-roots campaign with paid advertisements calling on Republican and Democratic House members to oppose the farm bill. It said the effort also included e-mail and social media campaigns, phone calls and two online videos that target a bipartisan list of representatives in 15 House districts, including Speaker John A. Boehner, Republican of Ohio, and Representative Collin Peterson, Democrat of Minnesota, who is the ranking member on the House Agriculture Committee.

“The 2013 farm bill has become little more than a vehicle for massive welfare spending and taxpayer-funded corporate cronyism,” said Tim Phillips, president of Americans for Prosperity. “Americans deserve better than this kind of government overspending and waste, and we’re going to make sure their voices are heard in Washington.”

On Monday, the Environmental Working Group, a Washington-based research organization, and representatives from a coalition of groups including the National Black Farmers Association and the American Enterprise Institute held a news conference to voice their concerns about what they consider overly generous farm subsidy programs.

“This is a terrible bill coming out of the House Agriculture Committee. It’s a windfall for already profitable farmers and insurance companies,” said Scott Faber, vice president at the Environmental Working Group. “If it fails, it will be because it contained too little reform and too much unbridled pork.”

While the groups are united in opposition to the farm bill, differences have emerged in some areas. Many of the conservative organizations oppose the food stamp program, which makes up the bulk of the bill, and want deeper cuts than what the House has proposed, while more liberal groups oppose cuts to the program.

The federally subsidized crop insurance program is one area where there is widespread agreement on the need for an overhaul. Taxpayers cover about 62 percent of the premiums for the program, under which farmers can buy insurance for poor yields, declines in prices or both. The result, the groups say, is that crop insurance has become more of a farm income support program than a system to protect farmers during disasters like the 2012 drought.

The policies are sold by 15 private insurance companies, which together receive about $1.3 billion a year from the government. The government also backs the companies against losses.

The House legislation would expand the crop insurance program by eliminating the $5 billion a year in direct payments to farmers and farmland owners who receive government checks regardless of whether they grow crops, and diverting some of that money to crop insurance. The bill would also create new crop insurance programs, including one to cover smaller declines in revenue or crop yields, and one that would cover 80 percent of cotton growers’ premiums.

“The bill should be rejected outright for its near-trillion price tag and its expansion of the government’s outsized and outdated role in American agriculture,” said Steve Ellis, vice president at Taxpayers for Common Sense, a budget watchdog group.