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."
Senators Vow Filibuster Over Planned Parenthood Cuts
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.
"The denial of simple birth control and family planning services may well increase both the number of unintended pregnancies and abortions in our country," former President Clinton said in a statement Wednesday.
Some Republicans, however, fear taxpayers' money is at least indirectly used for abortions, despite federal law that prohibits such use of taxpayer funds.
"Planned Parenthood is a private non-profit organization that should be sustaining itself, and their corporate business model relies on abortions," said New Hampshire Republican Sen. Kelly Ayotte in an interview posted on the Susan B. Anthony List website.
"Abortion yields 37 percent of all Planned Parenthood revenues by conservative estimates," she said. "I don't think it's appropriate to provide taxpayer dollars."
Three Senate Republicans -- Susan Collins of Maine, Scott Brown of Massachusetts and Lisa Murkowski of Alaska -- have publicly opposed defunding Planned Parenthood, saying the move goes too far.
And 41 senators -- all 39 Democrats and independent Sens. Bernie Sanders and Joe Lieberman -- have vowed they'll filibuster any bill that includes such a cut.
"The dangerous, ideological cuts that passed through the House are never, never, never going to pass the Senate," Sen. Chuck Schumer, D-N.Y., told a cheering crowd of Planned Parenthood supporters at a rally outside the capitol.
But many Republicans may be willing to take a stand on the Planned Parenthood rider or any number of other controversial measures, eager to appeal to their conservative base while also achieving greater budget savings.
"Our goal is real clear: We're going to fight for the largest spending cuts we can get, and the policy riders that were attached to them, because we believe that cutting spending will lead to a better environment for job creation," House Speaker John Boehner said Wednesday.