Republican leaders in the Arizona Senate, facing opposition from lawmakers in their own party, gave up plans to push through a $12.8 billion budget Thursday afternoon.

At least one GOP senator, Paul Boyer, has balked at a proposal in the budget to flatten Arizona’s income tax to 2.5%. The Glendale Republican told KJZZ’s “The Show” he’s concerned a massive reduction in tax collections will harm the finances of Arizona cities and towns, who receive a share of the revenue collected by the state. 

The flat tax has also faced opposition from K-12 public school advocates, who say a 4.5% cap on income tax liability undermines a tax hike on the wealthy that earmarked new revenue for education that voters just approved on the November ballot. Other community leaders also oppose the tax plan, which would cut an estimated $1.5 billion annually in state revenue.

Sen. Martin Quezada (D-Glendale) said he spoke to other Republicans on Thursday who also said they’d vote no on the GOP budget plan. 

“I know that there are several members who have problems with the budget for many different reasons,” Quezada said. “And I think their problems are all coming from different directions as well.”

Senate President Karen Fann (R-Prescott) alluded to facing opposition on multiple fronts in a statement before gaveling the Senate to a close for the week.

“We thought we were really, really close to getting a budget done today heads up, or heads down, whichever,” Fann said. “And over the last less than 12 hours, we're further apart now than we were; a lot of new requests, and a lot of new demands, whatever.”

“So it would be futile for us to try and put this up. ... We obviously need to get things back together again and figure out where we're going to go from here,” Fann added.

The Senate adjourned with plans to return no later than June 10. Like their counterparts in the Arizona House, which called it quits on Wednesday evening, Fann included a caveat that allows herself and House Speaker Rusty Bowers to call lawmakers back to the Capitol at any time between now and mid-June, as long as they give senators and representatives 24 hours’ notice.

Bowers faces the same troubling dynamic as Fann — Republicans hold one-vote majorities in both chambers. That means any one Republican can upend a budget deal as GOP lawmakers negotiate exclusively with their own caucus.

“Because they don't want to work across the aisle with Democrats on a budget proposal, they just simply didn't have the votes that they needed,” Quezada said. “And so their whole house of cards collapsed, and they're going to have to go back to the drawing board and come up with a different plan.”

It’s already the longest legislative session since 2013, when lawmakers didn’t wrap up their work until June 14.