Republican Fred Thompson has confirmed that he will enter the 2008 presidential race on 6 September.
Fred Thompson has delayed his official campaign launch for months
He plans to launch his candidacy on his website before going on a five-day tour to Iowa, New Hampshire, South Carolina, Florida and his home state, Tennessee.
The former senator and TV actor, 65, first hinted he might run several months ago, and his formal entry has been widely anticipated.
He has been consistently ranked among the top Republican contenders in polls.
Some commentators question whether he has missed a window of opportunity by waiting too long to enter the race.
News of Mr Thompson's formal announcement brings to an end many months of suspense for his supporters - and for his rivals.
Mr Thompson's backers have been urged to hold house parties to watch his webcast on 6 September, in which he is expected to outline his major policies.
While "testing the waters", he has been seen at many of the same events as his declared campaign rivals and maintained a visible presence on the internet with blogs and videos.
Many Republicans remain unexcited by the candidacies of frontrunners Rudy Giuliani, Mitt Romney and John McCain and have welcomed Mr Thompson's plain-spoken conservative image.
National polls and surveys in key early primary states - such as those targeted by his five-day tour next week - have placed him in the top tier of candidates.
Mr Thompson's preparations have been hit over the summer, however, with a series of changes among his campaign staff and questions raised over his background as a lobbyist.
His long delay in formally announcing also prompted some complaints that he was bending Federal Election Commission rules.
Mr Thompson has been seen at popular campaign events
In addition, Mr Thompson has raised significantly less money than some of his rivals.
That could make it harder for him to set up a multi-state campaign organisation.
He missed his goal of collecting $5 million (£3.5m) in his first month of fundraising by some way, banking only $3.4 million in June.
He has good name recognition from his acting career but some question whether he has sufficient political experience, especially on foreign policy issues.