Do they cosy up to the leader who may (despite all his denials) be the ultimate "king-maker"? Or do they concentrate all their fire-power on the man who is threatening to upset their chances of an outright win.
Remember: Nick Clegg was the bookies favourite to come first in the opening debate. In a sense he had nothing to lose in this encounter.
Whatever happened on the night, his profile was always going to receive a huge boost - as long as he answered the questions and avoided serious gaffes.
Gordon Brown's team believe he too had a good debate. Although the prime minister was always going to find it hard to come across as fresh and exciting, his advisers think he was comfortable and persuasive on the big issues.
David Cameron was inevitably going to be judged by the toughest standards, because many observers thought he would easily triumph in performance terms.
The Tory leader didn't do much wrong on the night, but nor did he manage to glide effortlessly away from his rivals. That didn't stop his supporters claiming victory.
There will be many who argue that Nick Clegg got away lightly. As Mr Brown and Mr Cameron traded blows, he had the space to present himself as the outsider - the man who might just take politics in a new direction.
Nick Clegg, David Cameron and Gordon Brown clash on live TV over MPs' expenses
Now Labour and the Tories will be frantically recalibrating the threat posed by Mr Clegg. Will a good performance in the debate really translate into extra votes or wins for the Liberal Democrats in a whole series of marginal seats?
Despite all appearances of the "prime ministerial debate", this isn't after all a presidential election. It's very hard for an American-style candidate to break through and overturn the loaded parliamentary arithmetic.
In the next debate in a week's time, Mr Clegg can expect a much tougher examination from his opponents. Mr Brown and Mr Cameron now need to find a way to deal with the Lib Dems. They can't simply be ignored.
Whether or not Nick Clegg emerges as a real contender for high office, one thing is certain. The debates that many predicted would be stultifying and rigid aren't quite turning out that way.
This election campaign has suddenly come to life.
• Sky News will be showing the second debate on Thursday, 22 April, which will look at foreign affairs. The final debate on BBC One on Thursday, 29 April will focus on the economy.
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