CLEVELAND - Liar. Garbage. Lock her up.

Republicans at their national convention are putting Hillary Clinton on mock trial, declaring her guilty and issuing sentences that include death by firing squad, in a remarkable display of political rhetoric gone wild. Even some Clinton haters say the vitriol has gone too far.

The focus on Clinton has sometimes upstaged what's supposed to be a weeklong celebration and promotion of Donald Trump. Instead of extolling the virtues of their nominee, Republicans have turned to increasingly crass slurs against his opponent.

One GOP delegate and adviser to Trump on veteran's issues, Al Baldasaro, took it a step further than the rest. He dubbed her a "piece of garbage" and suggested a punishment for alleged inaction during the 2012 Benghazi, Libya, attacks that left four Americans dead.

"Hillary Clinton should be put in the firing line and shot for treason," the New Hampshire state lawmaker said in a radio interview Tuesday.

Trump spokeswoman Hope Hicks said Trump and his campaign don't agree with Baldasaro's comment. The U.S. Secret Service said it was investigating.

On the streets of Cleveland, demeaning buttons for sale say "Life's a Bitch - Don't Elect One" and "Trump vs. Tramp." Others have been even harsher and more vulgar, with crude references to parts of Clinton's body.

Tony Ensminger, a 63-year-old selling buttons outside the arena, insisted "this was mild" compared to what Democrats said about former President George W. Bush.

Visceral disdain for Clinton has been palpable all week on the convention floor, where the go-to chant is "Lock her up." One former GOP presidential candidate, Ben Carson, drew a connection between Clinton and Lucifer, and Senate Majority Leader Mitch McConnell of Kentucky said scandals follow Clinton and her husband "like flies."

It's no surprise that Republicans are emphasizing Clinton's weaknesses more than Trump's strengths. The GOP's strategy for winning in November rests in part on the hope that voters dislike the Democrat more than the Republican.

Fifty-seven per cent of Americans in a July Associated Press-GfK poll said they viewed Clinton unfavourably, as did 64 per cent of independents who don't lean toward either party. Slightly more, 63 per cent overall, viewed Trump unfavourably; 62 per cent of independents.

It's those independents that Trump's campaign hopes it can peel off - if it can keep up a steady drumbeat of negativity about Clinton. Three-quarters of voters in the poll said their pick for president is motivated by a desire to cast their ballot against either Clinton or Trump.

Still, the criticism has triggered a backlash from some Republicans who say it's beyond the pale.

Arizona Sen. Jeff Flake, a Trump critic who's skipping the GOP convention, tweeted that Republicans "can make the case that she shouldn't be elected without jumping the shark." And Bill Pickle, a South Carolina delegate and talk radio host, said those doing the name-calling "sound and act like demons."

"I personally hate - I mean hate - all of the negativity," Pickle said. "What happened to professionalism, manners and humanity in our politicians and citizens?"

But most Republicans seem unperturbed. Wisconsin delegate Jim Geldreich said the stay-on-offence strategy was spot-on.

Clinton, meanwhile, has sought to use the negativity to her advantage. She sent out yet another fundraising appeal Wednesday night based on events at the convention, saying, "It's important to call out what we're seeing: What's happening at the convention is not normal and not acceptable."

Does it matter that this is the first time a major party is picking a woman to be its nominee? Johnny McMahan, 65, a GOP delegate from Arkansas, said he had no problem with a woman being president but some of his friends felt differently.

"They say women are too emotional to be president," McMahan said.

Though assailing the opposing candidate is standard fare at political conventions, this year the attacks have transcended policy positions and become intimately personal. The relentless spotlight on an opponent's supposed criminality is another departure.

Republicans maintain Clinton broke the law by sending classified information on her private email server.

Trump's campaign has worked to sow distrust by using vague insinuations against Clinton that are hard to prove or disprove.

On Wednesday, Trump campaign chairman Paul Manafort mused that Attorney General Loretta Lynch had "probably" given special information to Bill Clinton that helped his wife avoid criminal charges, during a brief airport meeting that Lynch has acknowledged was a mistake. He said the "Lock her up!" chant punctuating speeches at the conventions "probably reflects the attitude of a lot of people in America."

"They don't understand why justice wasn't done," he said.

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AP Polling Director Emily Swanson contributed to this report, along with AP writers Sam Hananel and Alicia A. Caldwell in Washington, Steve Peoples and Jill Colvin in Cleveland and Meg Kinnard in Columbia, South Carolina.