This, apparently, was the general-election speech Republicans needed all along.

Standing on a stage in New York on Wednesday, the party's presumptive nominee, Donald Trump, delivered a withering, 40-minute attack on his Democratic rival for the presidency.

He assailed Hillary Clinton as a "world-class liar." He referenced a much-discredited book suggesting she profited from corrupt governments. He attacked her "disastrous" judgment on voting for the war in Iraq.

Clinton, he said, lacks the integrity of a leader to serve in the Oval Office. "And the temperament!" he added, in a deliberate reference to the criticism Clinton used against him.

By Trump standards, this was a measured takedown — even if some of his more disparaging claims were embellished.

There were no winking exchanges with raucous supporters, no inappropriate jokes, no headline-grabbing, off-the-cuff tangents.

There was, in other words, no "side show," says Geoffrey Skelley, a political analyst with the University of Virginia's Centre for Politics. That's why, he predicts, "there will be a lot of feel-good reactions" from the party's top brass.

"It's exactly what they wanted from him, in terms of the tenor of his rhetoric and the focus of his message."

'You don't want yahoos just yelling'

While the speech echoed many of Trump's earlier messages about a "rigged" political system, Lisa Schiffren felt like she was hearing a rational Trump.

"It wasn't so much a departure, it wasn't a new voice," the former speechwriter for Vice President Dan Quayle says. "It was just put into a more coherent way."

Taking Trump out of a large stadium setting and removing him from the dynamic of a large crowd may have helped. The smaller room was a "smart choice" more befitting a serious policy speech than the large stadium crowds he typically favours, Schiffren says.

"You don't want a room that's full of yahoos just yelling."

The Republican presidential hopeful stepped up his attacks against rival Hillary Clinton 0:35

For concerned conservatives like Schiffren, Trump's use of a Teleprompter and decision to stick to the text of his speech were big reasons this latest address was effective.

"This is something most Republicans have wanted to see forever: a serious attack on Hillary, and on the substance."

Trump's speech was a grab bag of criticisms. There were attacks on Clinton's foreign policy. On her use of a private email server while secretary of state. On her response to the Benghazi embassy attack. On her stance on immigration and accepting Syrian refugees.

On the surface, it helped advance one of Trump's main themes, says political statistician Allan Lichtman: his "outsider" status. "He's saying he's going to change the system. The system is rigged. And [Clinton] is part of the system."

Trump may not have entirely broken with earlier habits, however: "When you look at what he said, it's typical Trump — filled with lies, exaggerations and half-truths."

On the attack

Trump claimed that Clinton accepted $58,000 in jewelry from the government of Brunei, for instance. But it did not, as he implied, go to her as a personal gift. Rather, it was a a standard diplomatic exchange that went to the State Department.

Lichtman also points out Trump's reliance on "completely bogus sources" to attack Clinton, including Clinton Cash, by controversial journalist Peter Schweizer, and Crisis of Character, a memoir by former secret service agent Gary Byrne.

The nonpartisan Association of Former Agents of the United States Secret Service has discredited Byrne's book, saying he "was too low-ranking" to plausibly bear witness to events he described. Schweitzer's book has also been widely dismissed for offering scant evidence to support assertions that the Clintons took cash from foreign governments, in exchange for favourable treatment while Hillary Clinton was secretary of state.

Non-partisan fact-checking website Politifact gives Trump a combined "false" or "pants on fire" score of 59 per cent, based on statements he has made thus far. Clinton has scored 12 per cent "false" or "pants on fire" statements.

Sticking to the script

But the sources of his claims may not matter very much to Trump's base, Lichtman says — not if the candidate can unite the Republican party against a common enemy.

"This speech will still help him within the Republican party. Because the one thing he shares with other Republicans, for sure, is their hatred of Hillary Clinton."

Trump's tendency to go off-script has made many Republican Party donors wary of the candidate. (David Becker/Reuters)

For now, at least, Lichtman says Trump has demonstrated an impressive ability to appear presidential, when he sticks to a script.

"Where he gets into trouble is when he goes off and thinks on his own," Lichtman adds, warning that Trump uncensored and uncaged is a whole other political beast — and not necessarily one the Republican Party can tame before November's general election.

"It's like the old movie King Kong," he summarizes. "Remember, Kong was fine behind bars. But as soon as Kong gets loose, just watch out."