On the surface, the budget battle in Washington hinges on the size of spending cuts, but it could be the policy on funding for Planned Parenthood and public television that ultimately lead to a government shutdown.
The difference between the two sides on spending cuts is merely a fraction of 1 percent of the overall budget. But the real sticking point seems ideological, in the form of policy riders that would significantly change government priorities.
"If this government shuts down -- and it looks like it's headed in that direction -- it's going to be based on my friends in the House of Representatives, the leadership over there, focusing on ideological matters that have nothing to do with funding this government," Senate Majority Leader Harry Reid said today.
House Republicans attached dozens of the controversial riders to their 2011 budget, which passed earlier this year and are the basis for ongoing negotiations.
The GOP riders would ban federal funding for Planned Parenthood, block Environmental Protection Agency efforts to regulate greenhouse gases and hamstring the carrying out of Democrats' recent legislative achievements, including a new Consumer Financial Protection Bureau and health care reform law.
One rider prohibits funds for new Transportation Security Administration employees. Another would zero out subsidies for public broadcasting, including PBS and NPR. Some riders even mandate foreign policy objectives, blocking NASA from collaborating with China and banning foreign aid to Saudi Arabia.
Conservatives insist some of the riders must be included in a bill or there's no deal.
"A bill without riders will not be passing the House," a senior House Republican aide close to the negotiations told ABC News.
"And fewer riders would mean more cuts," he added, when asked if Republicans would be willing to bargain on some of the more controversial items.
Democratic lawmakers participating in the deliberations have aired their frustrations over the number riders and their sweeping revisions to existing policies. They say major changes to policy should be debated publicly and considered separately from the general budget.
"What we can't be doing is using last year's budget process to have arguments about abortion, to have arguments about the Environmental Protection Agency, to try to use this budget negotiation as a vehicle for every ideological or political difference between the two parties," President Obama said Tuesday. "That's what the legislature is for ... to have those arguments. But not stuff it all into one budget bill."
One of the most controversial riders -- to eliminate Title X family planning grants for Planned Parenthood health centers -- could be a major point of contention in brokering a bipartisan budget deal.
Democrats and Planned Parenthood say the $363 million in taxpayer funds are essential to providing cancer screenings, sexually transmitted disease testing and general health services to an estimated 5 million low-income women who visit one of the 4,500 health centers around the country every year.