WASHINGTON — With a possible default on government obligations just days away, Senate Democratic leaders — believing they have a political advantage in the continuing fiscal impasse — refused Sunday to sign on to any deal that reopens the government but locks in budget cuts for next year.

The disagreement extended the stalemate that has kept much of the government shuttered for two weeks and threatens to force a federal default.

The core of the dispute is about spending, and how long a stopgap measure that would reopen the government should last. Democrats want the across-the-board cuts known as sequestration to last only through mid-November; Republicans want them to last as long as possible.

The Democrats’ demand shows a newfound aggressiveness. Previously, they had favored a so-called clean bill that would reopen the government and lift the debt ceiling without any policy changes attached. With Republicans on the defensive, it remains unclear whether the Democrats are using a negotiating ploy to raise the likelihood that any final deal will include their priorities as well as the Republicans’.