The 2016 presidential field expanded by two Monday with the addition of former Hewlett Packard boss Carly Fiorina and conservative neurosurgeon Ben Carson, adding diversity to the race for the Republican nomination.

They join three conservative US senators already on the campaign trail — Ted Cruz, Rand Paul and Marco Rubio. A sixth Republican contender, former Arkansas governor and television host Mike Huckabee, is expected to throw his hat in the ring Tuesday.

Former Florida governor Jeb Bush and current Wisconsin Governor Scott Walker, the current top Republican prospects, are likely to join the presidential fray in the coming months, making the GOP nomination race the most wide open in decades.

Democrats by comparison have had a sluggish start, with just Hillary Clinton and liberal Senator Bernie Sanders as announced candidates.

As the only woman so far in the crowded Republican field, Fiorina is confident her candidacy could be a counterweight to Clinton, the Democratic frontrunner who is also aiming to be the nation's first female commander-in-chief.

"Our founders never intended us to have a professional political class," Fiorina said in her campaign launch video, in which she turns away from an image of Clinton.

"We know the only way to reimagine our government is to reimagine who is leading it."

Fiorina sought to paint Clinton, who was secretary of state after serving as a first lady and then US senator, as "the personification" of Washington's political elite, who will do anything to perpetuate her family's power following her husband Bill Clinton's presidency.

"She and her husband have been in politics their entire lives," she said on a long conference call with reporters Monday.

Fiorina was a former advisor to defeated Republican presidential nominee John McCain in 2008. McCain tweeted that he is glad his friend Fiorina is running and "appreciate(s) her willingness to serve."

The 60-year-old executive headed Hewlett Packard for six years until 2005, when she was forced to resign after the company's share price plummeted.

In 2010, she ran an unsuccessful bid to unseat California's long-serving US Senator Barbara Boxer.

The Democratic Party swiftly blasted Fiorina Monday as a "job outsourcer" who was "one of the worst American CEOs of all time."

But the conservative-leaning Independent Women's Forum said Fiorina "brings business experience, a commitment to individual liberty and free market policies, and a confidence in all Americans that is a stark contrast to Hillary Clinton's identity policies."

– 'Not a politician' –

Carson, who entered the nation's political consciousness two years ago with a speech criticizing President Barack Obama's policies in front of Obama himself, is aiming to become the first African-American to win the Republican nomination.

The calm-demeanored Carson, who despite a childhood of "dire poverty" in Detroit, emerged as one of the nation's most respected medical professionals, has sought political "healing" and a renewed sense of compassion in America.

But his positions, including opposition to gay marriage, a "tough-love" approach to people receiving welfare, and his 2013 description of Obama's health care law as "the worst thing that has happened in this nation since slavery," have brought arguments that he would be too extreme a commander-in-chief.

"If God ordains that we end up in the White House… we are going to change the government into something that looks like a well-run business (more) than a behemoth of inefficiency," Carson, 63, said in his announcement speech in Detroit.

But he insisted it was a "blatant lie" that he would shutter vital programs.

"I have no desire to get rid of safety nets for those who need them. I have a strong desire to get rid of programs that create dependency in able-bodied people," he said.

Carson, who became director of pediatric neurosurgery at Johns Hopkins Childrens Center in 1984, has no political pedigree, but said his doctoral "experience in solving problems" would help him in the Oval Office.

He acknowledged his occasionally controversial comments, but said his candor is an asset.

"I'm probably never going to be politically correct, because I'm not a politician," he said to applause.

"Politicians do what's politically expedient. And I want to do what's right."