HARRISBURG — In one of Pennsylvania’s defining legislative battles this century, then-Gov. Ed Rendell won passage of a $1 billion tax package, and it still nags some Republicans that their leaders worked with Democrats to make it law, despite opposition from most members of the GOP’s legislative majority.

Twelve years later, an unwritten Republican rule inspired by votes like that under Rendell could be a key test as leaders of the Republican-controlled Legislature and Democratic Gov. Tom Wolf try to break a budget deadlock.

Simply put, the “majority-of-the-majority” rule means bills cannot get floor votes unless most Republican lawmakers support them. And many Republicans, particularly the most conservative, want to see their leaders adhere to that rule.

“I absolutely do,” said Rep. Justin Simmons, R-Lehigh. Otherwise, he said, “I would feel like they are selling us out.”

Sen. John Eichelberger, R-Blair, said he thinks his fellow Senate Republicans support the rule and should their leaders not observe it, they “would alienate a lot of people.”

That rule, plus much larger and more conservative Republican majorities than Rendell faced, could make it harder for Wolf to win passage of the package of tax increases he wants to finance a boost in public school aid and wipe out a long-term budget deficit.

In the 2003 tax vote, Democrats cast 90 of the 134 “yes” votes in the House and Senate. A combined 89 Republicans voted “no,” while 44 voted “yes.”

This year, a majority of the majority rule suggests that 77 Republicans will need to sign on, counting two House districts that are likely to seat a Republican in special elections on Aug. 4.

Still, Wolf’s drive for a roughly $2.5 billion tax increase in the first year — more than $4 billion once fully in force — could ultimately divide Republicans. Some are willing to support at least part of what Wolf wants, and a smattering of Republicans could join Democrats to deliver it.

In the Senate, Eichelberger estimated at least some Republicans will oppose a tax increase. Rep. Seth Grove, R-York, estimated 40 to 50 House Republicans, including him, will oppose any tax increase.

For Republicans, the calculus of a budget deal could get complicated.

The GOP may decide it’s worth allowing a floor vote on a tax package that Wolf supports if Wolf accedes to other elements of a budget deal that are dear to Republicans. As a sweetener, Republicans could try to force Democrats to supply the lion’s share of the politically sensitive “yes” votes necessary for the tax legislation to pass, similar to the 2003 budget fight.

So far, House and Senate Republican leaders already have ruled out passage of an increase of the sales or income tax. They left the door open to other tax increases sought by Wolf on Marcellus Shale natural gas production, sales of tobacco products and bank shares.

In any case, the days of “six or seven Republicans doing a deal with all the Democrats is not going to happen,” Senate Majority Leader Jake Corman, R-Centre, said.