SACRAMENTO - Gov. Gray Davis challenged Democrats on Tuesday to scale back their spending proposals and called on Republicans to support a half-cent sales tax hike, as budget negotiations inched toward a Sunday deadline that lawmakers said they do not expect to make.

The governor said he is "optimistic" an agreement will be reached on the spending plan by July 1, the start of the new fiscal year, but could not offer details showing progress is being made.

Meanwhile, talks among legislative leaders appeared to be going poorly, with lawmakers at an impasse. Senate President Pro Tem John Burton, D-San Francisco, stormed out of a private meeting with the governor and other party leaders shortly after it had begun Tuesday afternoon, but those who attended refused to explain why.

In addition, a novel effort bringing Republicans and Democrats together to break the state Capitol's rigid partisanship hit a stumbling block. Like the legislative leaders themselves, the members of a bipartisan Assembly group searching for solutions to the state's budget crisis cannot agree on whether to endorse new taxes or deeper cuts in core services, such as transportation, education or local government.

"The group is a reflection of the Legislature, and the issue comes down to taxes," said Assemblyman Keith Richman, R-Northridge. "It's a tough problem."

Davis, though, said he had recently come close to winning legislative leaders' backing of a budget plan but that it remained elusive.

"These meetings are like playing the accordion," he said. "Sometimes we are coming very close together. Sometimes we seem far apart."

Key lawmakers said hopes of meeting the June 15 constitutional deadline to approve a budget have faded.

"No, I don't think we'll meet that deadline at all," said Assemblyman Joe Canciamilla, D-Pittsburg. Along with Richman, Canciamilla has led the bipartisan group pushing the Legislature to develop a timely budget.

The failure of the group to bridge the difference over taxes illustrates the deep division between the two parties. It also shows the power of party leaders and special interests to intimidate legislators in the era of term limits, said Bruce Cain, a UC Berkeley political expert.

"It's very hard for individual members to buck the party line because somewhere along the line you'll get punished," he said. "The words of Jim Brulte have to be taken seriously."

Last week, Brulte, the Senate GOP leader from Rancho Cucamonga, threatened to campaign against any Republican legislator who votes for new taxes. On the other side of the spectrum, advocates for education and social services, among other causes favored by liberals, have lobbied Democrats hard to protect their programs.

Receiving special interest money and party support is more important than ever for lawmakers because term limits do not allow them to build up name recognition and other benefits of lasting incumbency. The risks of breaking party ranks appear to outweigh the benefits, from legislators' view, Cain added.

"The public is fickle. The public doesn't pay much attention to inside budget negotiations, and it hasn't embraced budget rebels in the past," he said.

From its inception in January, the bipartisan group prodded the Legislature to get serious about drafting a budget to cover a deficit now estimated by the legislative analyst to be near $30 billion for next year. The group succeeded on several fronts: The Assembly speaker canceled policy hearings for a week in March to focus on the budget and Republicans worked up their own list of specific cuts and backed a proposal to borrow $10 billion.

Canciamilla said the bipartisan group is continuing to meet to discuss budget issues, even if members cannot agree on an approach.