Jeb Bush (right) has struggled to escape the shadow of his brother, George. Photo: BBC

In a video released hours ahead of his official launch in Miami, the 62-year-old brother of ex-president George W Bush declared: "I'm ready to lead."

He also promised to protect the country's most vulnerable and remove the barriers to social mobility.

But doubts persist among conservatives in his party.

And early polling suggests that he has yet to dominate a wide field of Republican candidates.

In his latest video, entitled The Greatest Century, he strikes a very optimistic note, saying: "I see a great country on the verge of its greatest century, and I'm ready to lead."

Although his campaign becomes official on Monday, it's been no secret for many months and his team is well on the way to raising a $100m war chest.

During a tour of Europe last week, Mr Bush warned he would not waver from his core beliefs, even if some are unpopular in his party.

"I'm not going to change who I am," he said. "I respect people who may not agree with me, but I'm not going to change my views because today someone has a view that's different."

In a separate video, called Making A Difference, he champions the rights of women, ethnic minorities and the disabled.

"My core beliefs start with the premise that the most vulnerable in our society should be in the front of the line and not the back,'' he says.

"What we need is new leadership that takes conservative principles and applies them so that people can rise up."

He becomes the 11th Republican to declare, with Wisconsin Governor Scott Walker and Florida Senator Marco Rubio among his biggest rivals.

On the Democratic side, Hillary Clinton is the clear frontrunner. This raises the possibility in 2016 of another Clinton-Bush race like that of 1992, when Mrs Clinton's husband Bill beat Jeb Bush's father, President George HW Bush.

Declared presidential candidates

Democrat

• Hillary Clinton, former First Lady and Secretary of State
• Martin O'Malley, former governor of Maryland and mayor of Baltimore
• Bernie Sanders, independent senator from Vermont, caucuses with the Democrats
• Lincoln Chafee, former senator and governor of Rhode Island

Republican

• Jeb Bush, former Florida Governor
• Ted Cruz, Texas senator and conservative firebrand
• Rick Santorum, Christian conservative from Pennsylvania
• Marco Rubio, Florida senator since 2011
• George Pataki, former three-term governor of New York
• Ben Carson, author and neurosurgeon
• Carly Fiorina, former boss of Hewlett Packard
• Mike Huckabee, former governor of Arkansas
• Rand Paul, libertarian conservative Kentucky senator
• Lindsey Graham, South Carolina senator since 2003
• Rick Perry, former Texas governor