President George W. Bush and the US Congress are at an impasse over the budget for fiscal year 2008, which began on 1 October. And no one seems to have any idea how the log-jam will be broken, or to be working particularly assiduously at finding a path forward. Congress has passed a 'continuing resolution' to keep the government running at current spending levels until 16 November; the expectation is that a second such resolution will be needed to fund the government until around Christmas — a deadline that perhaps will force some real negotiations.

In the meantime, science funding is in limbo as Congress continues to pass spending bills in line with its budget outline — which provides about $23 billion more for discretionary spending than the president requested in February — and the president promises to veto any such bills.

Discretionary, as opposed to mandatory, spending includes money for virtually all research and development (R&D), and is the part of the budget for which Congress decides funding levels each year. Although, not counting defence, discretionary spending constitutes only about 20% of the nearly $3-trillion federal budget, it is the subject of most of the annual political wrangling. That's because it is much harder to change mandatory programmes such as social security, which spend whatever amount is needed to meet their legal obligation to provide benefits.

It is particularly hard to guess the final outcome for science this year, because the budget dispute is more about the size of the total pot of discretionary spending than about any specific programme. Historically, the overall level of discretionary spending has been the best gauge of what level of funding science will receive. As the science-policy scholar Daniel Sarewitz pointed out in a recent article in Issues in Science and Technology, for the past 30 years, R&D has amounted to between 10% and 12% of total non-defence discretionary spending. If one removes spending for the Apollo space programme, the percentage has been relatively stable for at least 45 years. Congress doesn't generally build a budget around science; as a popular but relatively obscure spending area, it gets whatever Congress can afford given the parameters of the overall budget.

Even though both political parties have made a point lately of portraying themselves as champions of science, science spending is still far from the most politically salient issue. The president has generally not attacked the science funding in the spending bills — not even in the case of the National Institutes of Health, for which the House of Representatives upped the president's proposed level by more than $1 billion. But if overall discretionary spending gets cut, science may have to take part of the hit.

It's too early to tell how the budget struggle will end. So far, it has apparently been in the political interest of both sides to draw lines in the sand. The president, who has never before vetoed a spending bill, wants to revive the Republican Party's reputation for fiscal responsibility and shore up conservative support in advance of the 2008 elections. The Democrats, in control of Congress for the first time in 12 years, want to demonstrate that they will increase funding for domestic programmes — especially when they've disappointed their liberal base with their inability to reverse course in Iraq.

The critical unknown is how swing voters, who determine the election outcome, will react. Voters tend to support spending in areas the Democrats want to increase, such as education, but they may also be concerned about deficits. Moreover, neither party wants to be blamed for causing a breakdown in the budget process, and it's hard to guess who the public in the end will see as unreasonable. The facts themselves can be confusing. For example, the president argues that his budget already increases discretionary spending by $60 billion over fiscal year 2007; the Democrats point out that the net increases are almost exclusively in defence and homeland security.

The politics within Congress are volatile as well. Hardcore conservatives have been angry with the president for years because spending and deficits have increased on his watch, and they would no doubt vote to sustain a presidential veto. Indeed, conservatives would probably be satisfied with a year-long impasse because it would keep spending at current levels. But other Republicans may be in no mood for a veto fight or a prolonged stalemate if either strategy makes it look as if their party can't govern. That's especially true given how small the disputed amounts are compared with the overall budget. Congressman David Hobson, a middle-of-the-road Republican from the swing state of Ohio, recently told Congress Daily: "I don't know who's advising him up there, but the president is really out of touch. It's too little, too late for him be a fiscal conservative … he wants $190 billion more for the war, but he's picking a fight over $23 billion?"

With that in mind, the most effective lobbying approach for the science budget might be to join forces with other interest groups and push for higher levels of discretionary spending. But science groups never do that, fearing — probably correctly — that getting in the middle of the broader, more partisan and ideological fight over fiscal policy would just create enemies, leaving R&D spending more vulnerable.

So as this year's spending battle grinds on, scientists and their advocates are once again on the sidelines — innocent bystanders hoping to avoid becoming innocent victims. Once decisions are made on overall discretionary spending, the lobbyists will go to work trying to protect the R&D funding that Congress has provided so far. To everyone's surprise, such last-minute scrambling worked earlier this year as Congress belatedly finished its work on the fiscal year 2007 budget. This time, with more interest groups geared up to fight for money after a year of Democratic control, R&D may face more competition if the $23-billion discretionary increase is trimmed. Lobbyists for R&D are likely to have a busy December.