HARRISBURG — With the state budget set to expire in a matter of days, the fundamental differences that remain between Democratic Gov. Ed Rendell and Senate Republican leaders have cast doubt on how soon lawmakers may approve a new spending package.

This week, Rendell laid out an ambitious set of legislation he said he expects on his desk before he signs a new budget, including $1 billion in new money for the state's deteriorating highway system and cash-strapped mass transit agencies. Senate Republicans, who control the chamber, maintained Wednesday that a new budget authorization bill is possible before Sunday, and should not be held up by other issues.

If a compromise is to be had, the Senate GOP may have to sign off on a borrowing package to generate close to $1 billion a year in new money for highways and mass transit. For his part, Rendell may have to wait until the Legislature's fall session to negotiate passage of a new electricity usage charge to fund alternative energy and conservation programs, Senate Democrats say.

If the sides agree to that scenario, legislators will be able to leave Harrisburg for their summer break, perhaps as early as Saturday, instead of dragging negotiations well into July, as Rendell has threatened to do by withholding his signature from a budget.

"It is conceivable and mechanically possible that we will have this done by Saturday," said Sen. Vincent J. Fumo, the Philadelphia Democrat who has been a key negotiator. "Now whether it's politically possible, that's a totally different question."

Senate Republican leaders would only say that they will look seriously at the centerpiece of Rendell's energy policy, which levies a charge on electricity usage, if and when it comes over from the House.

They began analyzing the transportation-funding package that passed the House on Wednesday, and did not dismiss the possibility of reaching an agreement on a version in the coming days.

They are not, however, happy about negotiating under the threat of a state shutdown.

"I don't think that's the kind of thing that should be done in a short cut fashion," said Sen. Dominic Pileggi, the Republican leader from Delaware County.

An extended budget impasse could mean furloughs of 26,000 state workers, and a freezing of state spending on items not deemed critical to the health and safety of the state's residents. For instance, Pennsylvania State Police patrols and health care subsidies for the poor would continue, while state parks and grant programs would shut down.