WASHINGTON - Death threats are part of the job for any American president, but Barack Obama has reportedly had more levelled against him than any commander-in-chief in history.

Concerns about his safety are mounting in the midst of a tense summer that has seen mobs of angry demonstrators showing up to protest Obama's health-care reform plans - including a man openly carrying a pistol on Tuesday in New Hampshire, where the president held a town hall meeting on health care.

It's legal to carry holstered weapons in the state, so long as they're not concealed. The man was also waving a sign that read "It Is Time To Water The Tree of Liberty," a reference to the Thomas Jefferson quote: "The tree of liberty must be refreshed from time to time, with the blood of patriots and tyrants."

Footage of the protester with his gun in a leg holster rapidly made its way to various news websites in the hours before Obama appeared, alarming video in a country that has seen four presidents gunned down in public and attempts made on the lives of 11 others.

"There is, and rightly so, a growing concern about the tone in American politics right now," Cal Jillson, a political scientist at Southern Methodist University in Dallas, said Tuesday.

"It started with Sarah Palin and the way the crowds responded to her last year, and continued with the tea party gatherings and now the town hall meetings on health care - there is an arc of rhetorical excess that I think is rightly worrying to people."

A new book, "In The President's Secret Service," says Obama receives as many as 30 death threats a day, 400 per cent more than those made against his predecessor, George W. Bush.

Its author, Ronald Kessler, has sent shockwaves through Washington with his allegations that the cash-strapped Secret Service is cutting corners, leaving the first African-American president in U.S. history particularly vulnerable.

"There's no question his life is in danger," Kessler, a veteran investigative journalist, said in a recent television interview. "Tomorrow, Obama could be assassinated ... simply because the Secret Service was not doing what it used to do."

The Secret Service has vehemently denied those allegations, but images of the armed New Hampshire protester amid suggestions Obama isn't being adequately protected are unsettling to many Americans.

"It's a matter of time until someone is shot. And if it's Obama, the country is going to explode in violence," wrote a commenter on the Washington Post's website on Tuesday.

Even Congress representatives have faced death threats this summer over health-care reform.

A Democratic lawmaker from Washington state received a faxed death threat a day after he described enraged town hall demonstrators as "a lynch mob." Representative Brian Baird says he's also received threatening phone calls, and was forced to cancel the rest of the town halls he'd scheduled.

The likeness of a Maryland congressman was hanged in effigy at a recent town hall, and protesters carried the fake tombstone of another Texas politician at a similar event.

Jillson says top Republicans are stoking some of the anger by making statements about Obama's health-care reform plans that are "demonstrably untrue."He pointed to Sarah Palin's recent Facebook message that alleged White House "death squads" would endanger the life of her disabled infant son.

"Any time you whip up people's emotions in a political setting, the result of that is uncertain and uncontrollable," he said.

"The Republican party is on a knife's edge here - they have certainly bloodied the Democrats politically on health-care reform, but they risk going too far."

But one former Republican aide says death threats are always part of the political landscape in the U.S., adding Americans shouldn't be overly concerned right now.

Stephen Hess of the Washington-based Brookings Institution once monitored the mail of Dwight D. Eisenhower in 1961, shortly after the president left office.

"Here is a man who is a retired two-term president, revered, a five-star general in the last just war, and he's getting dozens of dozens of death threats," Hess recalled with a laugh.

"We're a large country in which there's a certain percentage of nuts, often sick people. I don't think these threats have to do with the times in any special way. Most of these people frankly are just crazy, and they've always been around."

If anything, Hess said, the advent of information technology - e-mail, in particular - means threats are much easier to make now, and the mentally unhinged can level more of them despite having little chance of ever carrying them out.

"It's anonymous and it's instant," Hess said.

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