The Liberal Democrats are to launch their campaign to win the Westminster seat left vacant by the death of former foreign secretary Robin Cook.
Charles Dundas improved the Lib Dem's share of the votes in May
Their Livingston by-election candidate, Charles Dundas, came third to Mr Cook in May's general election.
Mr Dundas said he would put the spotlight on the economy, public services and the war in Iraq.
The Lib Dems, who opposed the Iraq invasion, improved their share of the vote in May.
The Scottish National Party and the Scottish Conservatives kicked off their campaigns on Wednesday, with their respective candidates Angela Constance and Gordon Lyndhurst.
Robin Cook's former election agent Jim Devine has been selected as Labour's candidate to fight the by-election.
The Greens, Socialists and UKIP are also fielding candidates to contest the Livingston seat.