By Iain Watson
Political correspondent, BBC News
Gordon Brown will hope economic figures can help his chances
Few could doubt Gordon Brown's determination to win the general election but many question his ability to do so.
So far, he has seen off three separate attempts by some in his own party to oust him, become far more assured against David Cameron at prime minister's questions, and seemed more energetic on the campaign trail.
But Labour still trail in the polls, and many party officials would regard a hung Parliament as something of a victory given that the party came fourth at the European elections last year.
The prime minister will have to overcome multiple challenges.
The first is to show a united front. Getting the formal endorsement of his predecessor Tony Blair last week will help, though his reappearance might invite less than flattering comparisons of style.
The second challenge is a rather difficult one to anticipate. As Harold Macmillan put it: "Events, dear boy, events."
So far we have had a BA cabin crew strike, with a rail strike in the offing too. The opposition will be able to present this as a "spring of discontent".
Continuity or change?
The third challenge for Gordon Brown is to persuade people that neither he nor his party have overstayed their welcome.
This is a tricky one, because he will have to defend Labour's record while also trying to argue credibly that his party, not the opposition, represents change.
We are told there will be forward-looking policies in the manifesto - from building a green economy to making the House of Lords democratic and reforming the voting system.
Labour might still be vulnerable to the charge that, after 13 years in office, it should have made more progress on some of these issues.
But there is one challenge which overwhelms all others - can Gordon Brown persuade us the economy is on the right track?
The gap between the Conservatives and Labour in the opinion polls began to narrow as Britain crawled out of recession.
New economic data will be released during the course of the campaign. If that shows Britain sliding back into recession, then credibility on economic management could be seriously undermined.
But if the economy appears to be performing even better than expected, voters might then feel they have the confidence to risk a change of government.
For Labour the ideal scenario is a continuing, steady climb out of recession which allows it to argue that the recovery is too fragile to withstand Conservative spending cuts.
When Mr Brown was chancellor, he liked to frame elections as a choice between Labour investment and Tory cuts. He can no longer do that.
With a plan to reduce the budget deficit by half, his challenge now is to convince voters that Labour cuts can, in some way, seem fairer than Conservative cuts.
Apart from all of these political challenges, Mr Brown faces a tough personal challenge.
Questions of character are likely to be significant in this election, with the first ever presidential-style TV debates between the party leaders.
Most polls still suggest the prime minister is less popular than his party, and the Conservatives have been relentlessly personalising the political battle as they regard Mr Brown as a liability for Labour.
But if the prime minister comes across in the debates as assured and experienced, he has a chance of persuading what will still be high numbers of undecided voters that he is the person to trust.
Since the start of the year, his aides have been undertaking what they privately call "a humanising strategy" on Mr Brown.
The advisers know voters do not naturally warm to the prime minister.
But if they can get him to combine competence with courtesy and seriousness with a smile, they hope their "victory" - stopping the Conservatives from forming a majority government and possibly leaving Labour as the largest party in Parliament - is within their grasp.