US President Donald Trump has decried the threat of political violence and called on the media to end its “hostility”, hours after authorities intercepted bombs sent to a news network and prominent Democrats who have been the targets of some of his sharpest barbs.

Mr Trump’s pleas for harmony came as law enforcement officials scrambled to find the perpetrator of the thwarted bomb attacks against former president Barack Obama, Hillary Clinton, CNN and others.

The scripted message was a dissonant one for the President, who has repeatedly blasted his political opponents as criminals and argued they will destroy the US if they win control of Congress in the midterms.

“We want all sides to come together in peace and harmony,” he said at a campaign rally in Wisconsin on Wednesday.

“Any acts or threats of political violence are an attack on our democracy itself.”

The President noted the unusually subdued tone of his remarks.

“By the way, do you see how nice I’m behaving tonight?” he said. “Have you ever seen this?”

Law enforcement authorities have ascribed no motive to the crime. Still, it has prompted immediate debate about whether increasingly hard-edged rhetoric has contributed to a potentially dangerous political climate.

Trump critics have blamed him for the tone. The President did not take any responsibility in Wisconsin.

Those “engaged in the political arena” must “stop treating political opponents as being morally defective”, he said.

He also referenced high-profile incidents in which conservatives have been accosted in restaurants and public spaces by political critics.

The media also has a responsibly to set a civil tone and to stop the endless hostility and constant negative and oftentimes false attacks and stories.”

Mr Trump has frequently labelled stories he doesn’t like as “fake news” and many reporters as “enemies of the people”.

At a rally in Montana last week, Mr Trump praised Republican representative Greg Gianforte, who pleaded guilty to assaulting a reporter, saying “any guy that can do a body slam … he was my guy”.

Mrs Clinton, who lost to Mr Trump in the 2016 presidential election, said it was a “troubling time” and a “time of deep divisions and we have to do everything we can to bring our country together”.

The President’s allies have pushed back against any suggestion he contributes to a toxic political atmosphere.

Senator Orrin Hatch, a Utah Republican, said: “I don’t see anything really wrong with the President. He’s in a tough position, attacked on all sides, and he ought to be able to express himself.”

Democratic party politicians and the head of broadcaster CNN have all but blamed Mr Trump’s inflammatory rhetoric for the wave of attempted bombings.

CCN president Jeff Zucker criticised the White House for a “total and complete lack of understanding” of the seriousness of its attacks on the media. His network’s New York bureau was evacuated for five hours following the discovery of an explosive device sent there.

Feelings were raw at the cable network because of what it believed was a reluctance by the Trump administration to discuss CNN as one of the targets of crude devices sent to political leaders, and the arrival of a fund-raising email that attacked CNN in supporters’ inboxes as the story unfolded.

“The President, and especially the White House press secretary, should understand their words matter,” Mr Zucker said.

“Thus far, they have shown no comprehension of that.”

Donald Trump has attacked his opponents with fiery rhetoric. Photo: EPA

The CNN package included an explosive device and an envelope containing white powder. It was addressed to former CIA director John Brennan, a frequent Trump critic.

Mr Brennan is actually a contracted analyst at NBC News, although the package’s street address was for the Time Warner Centre in New York’s Columbus Circle, where CNN’s offices are.

CNN shifted its broadcast from New York to Washington mid-morning on Wednesday after an alarm sounded and the building was evacuated.

Employees milled about in the streets, along with shoppers in the Time Warner Centre mall, which was also evacuated.

Mr Zucker was at the network’s Atlanta headquarters, where he helped direct coverage.

Less than two hours after the CNN offices were evacuated, Mr Trump’s campaign sent a fund-raising email to some supporters that specifically targeting CNN and urging recipients to fight back against the “fake news’ attacks and bias against hardworking Americans”.

Campaign chairman Brad Parscale later apologised, saying it was a pre-programmed message that was not caught before news of the explosive device came out.

Democrats also criticised the President.

“For years now, Donald Trump has been calling for the jailing of his critics and has lauded violence against journalists,” US Representative Bill Pascrell, a New Jersey Democrat, said.

“The danger of right-wing extremism cannot be ignored and more attention must be paid to it before even worse violence occurs.”

A recent Reuters/Ipsos poll found rising anger would be a factor driving voters in the November 6 midterm elections, when Democrats are seeking to regain control of at least one of the two chambers of Congress.

Politicians from both major parties have made condemning the harsh tone of politics part of their everyday stump speeches.

Senate Democratic leader Chuck Schumer and House Democratic leader Nancy Pelosi dismissed Mr Trump’s words, saying they rang hollow.

“Time and time again, the President has condoned physical violence and divided Americans with his words and his actions,” their statement said.

-with AAP