Mr Foster, the party's education spokesman, describes himself as the only candidate in favour of closer co-operation with Labour but stressed that his party should not seek to replace Labour on the left.
Using the campaign slogan "Ambitious for Power", Mr Foster made it clear that those who "play politics without seeking power lack self-belief".
His launch came as Liberal Democrat MPs spoke of a "general sense of anti-climax" after a hustings on Wednesday night. Almost all of the party's 46 MPs attended the hustings to hear addresses from the six candidates - Mr Foster, the bookies' favourite Charles Kennedy, Malcolm Bruce, David Rendel, Simon Hughes and Jackie Ballard. The official closure for candidatures is on Monday but, given their relatively high-profile launches, it does not seem likely that any of them will withdraw their bid.
One senior MP, still undecided as to whom to support, said the candidates had not done themselves "any justice".
The rival camps were hoping to sign up the 14 MPs who have not so far declared their allegiance. The main attention has focused on the "group of four" -Menzies Campbell, Paul Tyler, Nick Harvey and Archy Kirkwood - who are expected to back the same candidate.
Although the leader will be elected by a proportional-representation vote by the party's 90,000 members, endorsements from leading MPs could sway the opinion of local activists.
Mr Kennedy, the rural affairs spokesman, has so far been backed by 15 MPs, Mr Bruce, the treasury spokesman, by three MPs, while Mr Hughes, Mr Foster, Mr Rendel and Ms Ballard have the support of just two MPs each.Reuse content