TRENTON — For months, it was one of the quietest budget seasons Trenton has ever seen.

As the $30.6 billion spending bill moved through the Legislature, Democrats insisted they were serious about their spending and weren’t expecting Gov. Chris Christie to cut funding for the vulnerable constituents they were sending more money to.

“This isn’t a gimmick or a game at all,” Senate Budget Committee Chairman Paul Sarlo (D-Bergen) said the day the Senate passed the budget.

Behind closed doors, there was a battle brewing. In a meeting last week with Republicans, Sarlo admitted it was a “political budget,” according to attendees at the meeting. Sarlo denies ever saying that.

But from interviews with lawmakers and other Trenton politicians, a picture emerges of a weeklong political theater at its pinnacle. There was stagecraft on both sides, the various sources said, and some strategic serious mistakes by the Democrats who continue to underestimate the governor. It appeared Democrats, these sources say, had a plan: Blast Christie for every cut and claim victory for everything that he left in place. They thought they had him in a box.

It all blew up when Christie responded in a way they never expected. He cut deeper than what they added, about $900 million total, taking money away from some of the vulnerable and valued constituents that Democrats normally court. And he did it with a scolding lecture on spending restraint.

The Democrats, some whom had just worked to help Christie force public workers to pay for more of their pension benefits, responded in fury. Senate President Stephen Sweeney (D-Gloucester) called Christie a “bastard” and “cruel man” and many other spicy things, which, when it comes to public denunciations of a Jersey governor might even stand out as raising the bar.

“If you try to lie or manipulate or outfox this guy, he’s coming back at you with a nuclear vengeance,” said Sen. Kevin O’Toole (R-Essex). But in the end, both sides got some political fodder: a formula for demonizing each other as the election season heats up.

Some lawmakers and officials spoke on the record for this story, others, wary of retaliation, asked for anonymity.

From the beginning, this budget was not drafted in the usual Trenton way, and in retrospect some Democrats think they may have missed opportunities to get some of what they wanted. In most years, the governor submits a budget and then the haggling starts, one side gives a little and the other makes concessions. The budget becomes a bargaining chip during every other negotiation.

That didn’t happen this time.

Unlike years past, leaders in Trenton spent only a week working on the budget, a process that included almost no communication between legislative Democrats and the governor.

Christie, Sweeney and Assembly Speaker Shelia Oliver (D-Essex) spent months hammering out a deal to pass legislation to force public employees to pay more for health and pension benefits.

When the bill passed the Legislature on June 23, they finally took up the job of hammering out the state’s roughly $30 billion budget.

There were no negotiations. No deals were brokered. Just tersely worded news releases from both sides.

Democrats couldn’t agree on how to approach the budget, said Sen. Raymond Lesniak (D-Union). He said some supported authoring their own budget while others wanted to take a list of requests to the governor and try to work out a deal.

“There were others who thought that we could get stuff by negotiating with the governor,” Lesniak said. “I felt that our ability to do what we believe is right is not just to get stuff for our districts.”

Ultimately, the Democrats decided to get behind their own budget. It added a couple extra hundred million for suburban schools, $50 million for cops in urban areas and reversed his cuts to Medicaid programs for the state’s working poor.

Democrats expected Christie to remove big chunks of spending and leave a few pieces that they wanted, like the money for police in crime-plagued areas. Democrats had also attempted to write the budget using complicated language in a way that made it difficult to undo their changes to Medicaid.

Before the budget even made it to Christie’s desk, Democrats were already planning to come back this week and vote to override his vetoes. The override votes, which have virtually no chance of being successful, would force Republican lawmakers to vote against each individual reduction. With every lawmaker up for re-election in November, the campaign attack ads would write themselves.

Sweeney insisted the budget not be the product of negotiations, sources familiar with caucus discussions said.

Even during the weeks of heated negotiations on the pension and health benefits bill, the topic of the budget didn’t become a bargaining chip, the governor said.

Lesniak called it a “miscalculation” for Democrats to not include the budget in negotiations.

Many Democrats said they assumed Sweeney was talking about the budget privately with Christie, laying the groundwork for compromises. In the end, they were surprised to learn he wasn’t kidding when he insisted he wasn’t, sources confirmed.

The day the Legislature passed the Democratic budget, Christie pulled Republican legislative leaders and staff into his office. Around two folded tables with white table clothes and boxes of pizza, they talked about how to respond, those with knowledge of the meeting said.

Democratic staffers down the hall waited nervously; what would the governor do? He could call their bluff and veto the whole budget, leading to a standoff and shutting down government. Christie said later at a news conference to announce his line-item vetoes that they had made no plans to shut down government but had seriously considered a total veto.

When Christie handed down his emaciated budget that chopped $900 million out using the line-item veto, Democrats were shocked by how deep he cut.

“I think we’re all stunned that he would be so cruel and mean-spirited,” Lesniak said.

Some Democrats now admit they expected the governor to make some reductions but that they never expected him to go that far.

Christie cut $139 million in aid to the state’s most financially strapped cities, money he had originally included in his own budget. He cut $25 million from college tuition grants for the poor, a move that seems to counter his oft-repeated position that higher-education is a priority once the economy starts to recover. He cut staff salaries in the Senate and Assembly, an unprecedented move for a governor to go after legislative staff. He cut $250,000 out of the Office of Legislative Services, a division fully funded in the budget he presented in February but that drew his ire in the interim by disagreeing with his revenue estimates.

“We were $289 million apart,” Sarlo said. “He went and cut things that we didn’t touch. It was very vindictive and mean-spirited.”

Democrats added money and cut other funds that were incompatible, like reducing oversight funding for aid to municipalities, said Assemblyman Declan O’Scanlon (R-Monmouth), who is the ranking Republican member of the Assembly Budget Committee. Christie cut aid to the state’s poorest cities because the oversight office had been cut by Democrats, O’Scanlon said.

“These things were designed to bait the governor,” he said. “The budget was designed to be political from the start. This budget was designed for the sound bites that you’re hearing now, that the governor is a cruel and insensitive human being.”

Democrats counter that the budget isn’t about their talking points for this year’s legislative elections, but about Christie garnering more credentials for a presidential run.

“This, to me, sends to a message that he’s running for president because he’ll never get re-elected in New Jersey with this kind of policy,” Sen. Barbara Buono (D-Middlesex) said.

Democrats had included increased money for suburban schools in an effort to force the veto and upset Republican constituencies. But instead of cutting the school money, Christie left it and cut elsewhere, Kyrillos said.

“There are some things that he would have liked to have kept intact, but he decided otherwise because the Democrats gave him the opportunity to be able to make the decision to spend more money on suburban school aid,” Kyrillos said. “Instead, opting for a one- or two-day political statement, they completely lost control over items they say are priorities for them.”

In the end, Republicans say it was just a poorly calculated decision by the Democrats.

“They played chicken with the wrong guy,” said O’Scanlon.