Labour's rivals are eyeing the possibility of a snap general election, when Tony Blair steps down next year.
Mr Brown is the favourite to succeed Mr Blair
The Liberal Democrats have reshuffled their front bench, and appointed a manifesto chairman, to put them on an election footing.
They believe there could be an election as early as October 2007.
Tory leader David Cameron said voters should be given the chance to vote on the new Labour prime minister "as soon as is reasonably possible".
The government does not have to hold another general election until 2010, but Labour chairman Hazel Blears has already warned activitists it may be as early as spring 2008.
The Lib Dems believe it could be as early as October 2007, if Chancellor Gordon Brown succeeds Mr Blair as prime minister - as is widely expected - when he steps down in the next nine months.
They have already made Steve Webb chairman of the manifesto team in preparation for a possible October election.
Sir Menzies said: "A snap general election next year is a real possibility. With Steve Webb taking on the vital task of co-ordinating our election manifesto, I have had the chance to promote one of the best and brightest within our Parliamentary ranks."
Lib Dem chief of staff Norman Lamb has been appointed health spokesman and campaigns chairman Edward Davey replaces him as chief of staff.
Susan Kramer takes on the trade and industry portfolio and hands the international affairs brief to Lynne Featherstone.
Meanwhile, David Cameron said in a radio interview that the new Labour leader should be subject to an immediate test of public support.
"Tony Blair said at the last election he would serve a full term, so when he goes, no subsequent Labour prime minister can really in their heart claim to have a mandate from the British people," he told Classic FM.
Sir Menzies has made Norman Lamb health spokesman
"So it would be right actually to hold a general election as soon as is reasonably possible, because the British people thought they were electing Tony Blair.
"He's off. Someone new is coming. They need a mandate."
In a letter to party activists in December, Ms Blears said a general election may be "less than 16 months away".
She added: "The Tories are making a comeback, the next general election will not be easy.
"A swing against Labour of just 1.3% could see the Tories forming the next government."