The old, bilious Donald Trump came roaring back at a rally in Indianapolis on Wednesday, lashing out extravagantly at his rivals and the media but directing his wildest ire at the leaders of his own party for running what he lambasted as a “crooked” and “rigged” nominating system.

Jettisoning the relative decorum and restraint he showed in Manhattan on Tuesday evening after his blow-out win in the New York primary, the Republican frontrunner returned to his old campaigning form excoriating the “dummies” running America and scorning the few protestors who attempted to interrupt him (one in a Captain America suit and a Donald Trump mask).

That he was in Indiana, which votes on 3 May, was itself notable. Five more eastern states, including Pennsylvania and Connecticut, will vote first next Tuesday. But while Mr Trump should perform strongly in those states, the outlook in Indiana is altogether sketchier.

Experts believe Indiana could be the place that decides Mr Trump's future (AP)

At best for Mr Trump it is a toss-up state, which means it has suddenly emerged as crucial not just for him but also for his two rivals, Senator Ted Cruz and Ohio Governor John Kasich. If the New Yorker can win most of its delegates, he will still have a narrow path to eke out a majority of delegates nationally to seize the party nomination before the convention in July.

But if he doesn’t that path will become extremely tenuous. Without Indiana, even winning California on 7 June will probably not be enough to get him to that magic 1,237-delegate milestone. That in turn could spell a floor fight at the convention and an opportunity for the party that doesn’t want him to deny him the nomination.”

“Indiana is going to be very important…I wasn’t supposed to be here today,” he declared from the stage, acknowledging what even the state itself may been have slow to recognise – its unprecedented importance in this year’s primary marathon that is still far from being over, at least for Republicans. “I was supposed to be here in two weeks but I had to come early.”

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The high stakes were the ignition that sent Mr Trump back into orbit inside a cavernous pavilion at the state fairground, dashing pundits’ predictions that he would morph after New York into a gentler, more traditional politician. That, however, would mean setting aside the campaigning formula that worked so well for him in most of the states he has already won.

Mr Trump means, it seems, to carry on as the crockery-smashing candidate, who draws strength precisely from not behaving like a politician and promising to remain that way as president.

He was back vowing to build his wall, promising to obliterate Isis and reintroduce torture for terror suspects. “I love it,” he said of waterboarding, drawing boos from some protestors who had yet to be ejected. “The only thing is we should make it much tougher than waterboarding. Believe me we will get rid of Isis so fast that your heads will spin, so fast.”

Mr Trump has said the plant would not be shifting to Mexico if he was president (AP)

He meanwhile relaunched his scathing criticism of the Republican Party hierarchy which he contended is presiding over a system of assigning delegates that has become partially disconnected from how people actually vote in the primaries and caucuses and is corrupt.

“It’s a rigged, crooked system that decides things so that the bosses can pick whomever they want so that people like me can’t run and can’t defend you …it’s rigged system,” he vented to loud cheers. “I call her crooked Hillary,” he went on, referring to Hillary Clinton whom he may meet in a general election. “This is a crooked system, and we are going to get a change.”

As for the media, confined to a pen in the middle of the pavilion floor: “They are the most dishonest people in the world,” he railed, eliciting loud boos. “They are the worst. Honestly..Do we like the media?” (No! from the crowd). “ Do we hate the media/” (Yes!)

Part of the trouble reading Indiana stems from the absence of any reliable state-wide polls. The state has much in common with other Midwestern states he has won, including Michigan, but has similarities also with Wisconsin, the state where he was humiliated by Mr Cruz.

While the anti-Trump movement extended to the entire Republican apparatus in Wisconsin, from Governor Scott Walker on down, that kind of unity does not appear to be holding in Indiana. All three candidates are still vying for the endorsement of its sitting Governor Mike Pence. Indeed Mr Trump’s first stop in Indianapolis was Mr Pence’s office.

Yet, there will be headwinds, notably from one of the most conservative political committees out there - or Super PACs – the Club for Growth Action which has vowed to spend in excess of $1m on television advertising attacking Mr Trump in the state while supporting Mr Cruz.

“We’re gearing up to have a major presence there,” warned David McIntosh the group’s leader and a former US Congressman from Indiana. “It’s key that Cruz wins to stop Trump from getting the nomination.”

Like others, he sees the leverage his native state now has to block Mr Trump’s path to the nomination. “Indiana is going to be at that pivotal moment to decide what happens next in this presidential campaign that likely is to go all the way to the convention,” he said.

Yet the contagious thrill that has been palpable in so many of Mr Trump’s rallies in other states seemed to be taking hold again at the Indianapolis event.

“He is the right man at the right time,” offered Bryan Poynter, 49, who took the afternoon off selling houses to come to the rally. “He is not perfect and he doesn’t speak like the other candidates do and I like that and I hate it at the same time. I wish he was more polished sometimes. You can be direct without, you know, coming off as childish.”

Is he confident that his candidate can win? “It’s hard to tell,” he replied, adding that between hs rivals he expects Mr Kasich to “do extraordinarily well,” in the state.

Even seasoned professionals in Republican fold in the state confess to a deep uncertainty about how the next two weeks will play out. It’s uncharted territory for them too in part because usually by the time Indiana gets to hold its primary the races for the nominees of both parties have long been settled. But this year that is not the case.