Jeb Bush claims he is toughest Republican opponent for Hillary Clinton
A feisty Jeb Bush sought to regain his footing in the Republican presidential race, casting himself as the strongest election opponent for Democratic frontrunner Hillary Clinton.
The former Florida governor took advantage of a policy-focused debate to detail positions on the economy and immigration.
The televised debate is the last for the Republicans until mid-December, and could help shape the course of the campaign into the winter.
Voters are beginning to pay more attention to the White House race less than three months before the lead-off Iowa caucuses start the state-by-state nominating contests.
Billionaire Donald Trump's grip on the Republican field has been challenged in recent weeks by retired neurosurgeon Ben Carson, another outsider.
As Mr Carson has risen in the polls, however, he has faced a flurry of questions about his biography, which has been central to his connection with voters.
He sought to steady his campaign in the debate, swatting away questions about the veracity of his celebrated biography detailing his rise from poverty to become an acclaimed neurosurgeon.
"We should vet all candidates," Mr Carson said. "I have no problem with being vetted. What I do have a problem with is being lied about."
While pieces of his background had been challenged earlier in the campaign, the questions ballooned last week.
CNN reported it could not find friends or confidants to corroborate the story, told in his widely read autobiography, of his unsuccessfully trying to stab a close friend when he was a teenager.
Later in the week, Politico examined Mr Carson's claim of having been offered a scholarship to attend the US Military Academy,.
And the Wall Street Journal said it could not confirm anecdotes told by Mr Carson about his high school and college years.
The debate in Milwaukee opened with a narrow focus on economic policy, with moderators from Fox Business News allowing the candidates to deliver lengthy answers and avoiding attempts to get them to engage with one another.
Mr Bush entered the debate in need of a strong performance to ease the anxiety of donors and other supporters.
He sharply criticised Mr Clinton for defending President Barack Obama's economic policies, saying the nation must do better.
Mr Bush also stood by his call for immigration policies that would allow some of those in the US illegally to find a path to legal status.
He criticised Mr Trump's call for mass deportations as an impractical plan that would hand Democrats a talking point as they seek to appeal to Hispanic voters.
''They're doing high fives in the Clinton campaign when they're hearing this," Mr Bush said.
He avoided tangling with fellow Floridian Marco Rubio, a shift in strategy from his poor performance in the last debate.
Still, Mr Rubio faced criticism from some rivals about whether he is a true conservative given his calls for a child tax care credit and increasing military spending.
"We can't even have an economy if we're not safe," Mr Rubio said.
Mr Rubio's call for increased military spending was backed by Mr Trump, the real estate mogul who has led the Republican field for months.
Drawing a contrast with Democrats, the candidates voiced opposition to raising the minimum wage, casting it as an impediment to job growth.
Also on the stage were Texas senator Ted Cruz, businesswoman Carly Fiorina, Ohio governor John Kasich and Kentucky senator Rand Paul.
Missing from the line-up were New Jersey governor Chris Christie and former Arkansas governor Mike Huckabee.
Both were dropped from the top-tier debate with low poll numbers in national surveys, sparking criticism for the way networks hosting the debates have decided participation.
They instead appeared in an undercard debate, along with Louisiana governor Bobby Jindal and former Pennsylvania senator Rick Santorum.