Former Republican House Speaker Newt Gingrich has announced he will run for the presidency in 2012.
Mr Gingrich, who left elected office in 1999, becomes the best known of what analysts consider a lacklustre field of declared Republican candidates.
Other major potential hopefuls have yet formally to enter the race.
Mr Gingrich is best known for leading a team of conservative Republicans who won control of the US House of Representatives in the 1994 election.
Mr Gingrich, 67, first announced his candidacy on Wednesday through his account on the video sharing website YouTube.
“I believe we can return America to hope and opportunity, to full employment, to real security, to an American energy programme, to a balanced budget,” Mr Gingrich said in the video.
Mr Gingrich added: “We’ve done it before. We can do it again.”
In order to “get America back on track”, he said US citizens would have to “talk together, work together, find solutions together and insist on imposing solution on those who don’t want to change”.
“There’s a much better American future ahead,” Mr Gingrich said.
On his Twitter feed on Thursday, Mr Gingrich advised supporters to watch television personality Sean Hannity’s programme on Fox News on Wednesday evening.
His entrance into the race has been widely anticipated.
“Start Quote
He’s a big figure in a potential field where other candidates are either exciting but lack credibility or serious but dull”
End Quote Mark Mardell North America editor
Mr Gingrich is seen as a font of conservative policy ideas and controls a broad network of non-profit organisations and business ventures.
But he is twice divorced, and in the 1990s – while he was leading the charge to impeach President Bill Clinton in connection with his affair with a White House intern – he had an extramarital affair with the woman who became his third wife.
Socially conservative voters influential in the Republican primary race could be turned off by that past, analysts say.
Test the waters
Other likely Republican 2012 candidates include former Massachusetts Governor Mitt Romney, former Alaska Governor Sarah Palin, former Arkansas Governor Mike Huckabee, former Utah Governor and US ambassador to China Jon Huntsman, former Minnesota Governor Tim Pawlenty and real estate mogul and reality TV show star Donald Trump.
Of those, only Mr Romney and Mr Pawlenty have taken the first official step toward a candidacy, the formation of exploratory committees to test the waters and begin raising money.
The race for the Republican Party nomination has been slow to take shape this year, even though Democratic President Barack Obama is seen as vulnerable; the first ballots will be cast on 6 February in the Iowa caucus.
Of the candidates deemed most credible by the US news media, only Mr Pawlenty attended the first Republican presidential debate held last week in South Carolina.
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Gingrich annouces White House bid