Mitt Romney has said he is running for US president in 2008, making him the first of the Republican front-runners to make his candidacy official.
Mr Romney is positioning himself as a social conservative
Casting himself as an outsider, the Massachusetts ex-governor said the US needed "innovation and transformation".
Correspondents say he may struggle to beat Rudolph Giuliani and John McCain for his party's nomination.
Meanwhile, top White House adviser Karl Rove has warned that the long run-in to the 2008 ballot could turn voters off.
If elected, Mr Romney would become the country's first Mormon president.
Many Americans are suspicious of the religion, with a new poll suggesting one in four people would not vote for a Mormon for president.
But Mr Romney has made overtures to the Christian conservatives who make up a solid portion of the Republican party's base.
He is positioning himself as a social conservative to try to capitalise on Republican concerns that Mr McCain and Mr Giuliani are out of step with the party on issues such as abortion, gay marriage and gun control.
In a speech at the Henry Ford Museum in Michigan - a state his father governed in the 1960s - Mr Romney said he was for lower taxes and against abortion and gay marriage.
And he said the country needed a strong defence.
"As we stare at the face of radical violent jihad and the prospect of a nuclear epidemic, our military might should not be subjected to the whims of ever-changing political agendas," he said.
"The best ally of peace is a strong America."
He also addressed the issue of his religion obliquely, saying: "I believe in God and I believe that every person in this great country, and every person on this grand planet, is a child of God."
Mr Romney, 59, was a successful businessman who took charge of the 2002 Salt Lake City winter Olympics when planning was in crisis.
He is credited with turning the operation around and delivering a well-run sporting event.
He went on to become governor of the state of Massachusetts, stepping down early this year after one term. Correspondents say he is considered an effective fund-raiser.
The Republicans and the Democrats will chose their presidential candidates in a series of party primaries and caucuses early next year before the two nominees face off against each other on 4 November 2008.
Experts say the contenders for the primaries will need to raise more than a million dollars a week to keep in the running, the BBC's Justin Webb in Washington says.
Mr Rove, President George W Bush's senior adviser, has said he is worried the candidates will wear out their welcome with the American people well before that, our correspondent adds.
Favourites for the Democratic nomination include Senators Hillary Clinton and Barack Obama, and former Senator John Edwards.
The race has started unusually early this cycle, experts say, because George Bush leaves no heir apparent and is unpopular, limiting his ability to continue setting the national agenda.