Over decades of intense disputes about the federal budget, Republicans and Democrats have shared a set of expectations for what government spending and taxes would look like in the future.

The baseline, as it is called, assumed that Congress would operate on a form of autopilot. Spending would climb every year, in part to keep up with inflation, and tax rates would go up or down based on laws already on the books. The cost of policy changes would then be assessed against this rough sketch of the country’s fiscal trajectory.

Senate Republicans are preparing to upend that standard with the broad tax and health care bill they are trying to muscle into law. They want to create a new baseline of their own and wipe away much of the stated cost of the bill. The accounting gambit has angered Democrats and depressed independent budget experts. It could also soon force a reckoning between Republicans’ ambitions for cutting taxes and the Senate’s longtime parliamentary rules.

“We do worry about the precedent that this sets for both parties to say that this thing, whether it’s a spending increase or a tax cut, doesn’t cost what traditional scoring conventions, the conventions we’ve been using for decades, say it costs,” said Andrew Lautz, an analyst at the Bipartisan Policy Center, a think tank. “You worry about opening Pandora’s box when it comes to scoring.”

The Republican effort focuses on changing how scorekeepers represent the cost of tax cuts. Under traditional standards, extending tax cuts beyond their scheduled expiration is treated as passing a new tax cut — and therefore a new cost to the budget. In wonky budget-speak in Washington, this is called the “current law baseline.”

Because much of their legislation is dedicated to extending temporary tax cuts from 2017, Senate Republicans have become preoccupied with this accounting practice. Just maintaining the 2017 tax cuts after this year would cost roughly $3.8 trillion over a decade under the current law baseline.