Both Nick Clegg and Nigel Farage should be able to profit from their two-legged encounter, which kicks off this evening, but the event will also highlight the challenges which both parties face in winning this argument with the British public as a whole. For Ukip, the event is a sign of their growing mainstream political status. Challenges to debate tend to be issued by underdogs but, as with Gordon Brown issuing a challenge to opposition leader David Cameron in 2010, the office-holder can be the underdog.
The Deputy Prime Minister challenging a party leader with no Commons seats at all reflects the political reality that the Liberal Democrats are the underdogs in the European elections. In 2009, the Lib Dems came fourth and Ukip second, even when both parties were in opposition. In 2014, Ukip is confident its bid to top the poll can succeed, while a realistic Lib Dem ambition will be to secure enough support to hold off a Green challenge for fourth place.
Clegg challenged Farage to these debates about Europe, but this means that he has challenged Ukip to a debate about immigration too. Nigel Farage will certainly want to make this a debate about immigration. The Ukip leader believes that the public care more about immigration than Europe. He is right about that - especially when it comes to those considering voting for his party.
For both Clegg and Farage, the debate offers an opportunity to play to their respective bases of support - on both issues. The question is whether either can reach beyond that. About one in four people will always prefer Clegg's unambiguously pro-European "party of in" stance to the UKIP bid to exit the club. These voters are comfortable with current levels of immigration too, and often struggle to see why anybody should worry about the benefits that it brings. About one in four people are convinced that Nigel Farage is right that Britain has no choice but to leave, and are determined to see immigration at the lowest possible level. Most of this group would want to close the borders if they could.
But that leaves most people open to persuasion, neither committed to Team Nick or Team Nigel. The average British voter is not convinced by the case for the EU, nor persuaded that we would be better off out. In a referendum tomorrow, the vote would be pretty evenly split. While almost half of voters say they know for sure how they will vote in a referendum, most say they could change their minds. When asked about Britain's long-term position, only 28 per cent want to leave the club. Meanwhile, 55 per cent say they would like to stay in the EU, though most of this group would like to see David Cameron successfully negotiate to reduce Brussels' powers. Seventeen per cent don't know. Beyond the "pro" and "anti" European tribes, the British public is open-minded and up for grabs.
Clegg's challenge is to make the liberal argument make sense beyond those groups - particularly graduates, younger voters and Londoners - who begin strongly disposed to his side of the argument. Perhaps less noticed is that Ukip face a similar challenge, or a reverse mirror image of that facing pro-European liberals. Though Ukip's self-image is one of offering a populist common sense challenge to the political elite, authoritative new research, by the academics Rob Ford and Matthew Goodwin, shows that they are not a "catch-all" populist party, but rather the party with the most distinctive sociological profile of all. The Ukip vote is older, more male, more working-class and more likely to have left school at 16. The party's messages resonate with these "left behind" voters - but have much more trouble connecting with Britons born after 1966.
Tactically, it could be argued that neither Clegg nor Farage needs to reach out much when it comes to a pre-European elections face-off. After all, only one-third of the electorate will participate in May's vote. Preaching to the already-converted can be enough in a low participation, low stakes and low salience election.
But, strategically, both men should realise that this is not enough if they want to engage and try to win the bigger, contested arguments about the national interest: whether Britain is better off in or out; whether the EU can be reformed to better reflect our interests and values, or whether that is a quixotic quest; whether the British could decide that EU free movement is a two-way street worth keeping, or at least a price worth paying.
These issues will not be settled by this initial skirmish at the European election hustings, nor by the European elections themselves. Both Clegg and Farage are interested in winning the public argument about Britain's future: they each need to try to reach out to the unpersuaded majority too.