Nick Clegg is escalating the coalition slanging match

Nick Clegg today takes the coalition's pre-election slanging match to a whole new level. In a speech in London, the Deputy Prime Minister is expected to hit out at the Conservative party's target of reducing net migration to the tens of thousands, saying it has prevented the coalition government from working effectively to improve border controls.

If you believe the polls, immigration is the single most important battleground in British politics. YouGov's tracker on the topic shows that this March marked the first time in four years that immigration matched the economy as what voters thought was the most important issue facing the country. Voters' concerns about unchecked immigration have also been central to the rise of Ukip, whose “people's army” now stands at the gates of some key Conservative seats, and who drove Clegg's Lib Dems into painful defeat at the European Elections this May.

From a policy perspective, Clegg's central point is a good one. If a government's focus is solely on net migration, their policy could be subject to two key flaws.

First, the net migration figure includes the arrival and departure of Britons. There's nothing to suggest that the Conservatives have been driving British people away for a sunny retirement in Spain to fiddle the stats, but it shows that the figure doesn't necessarily address much of what concerns the average voter about large-scale migration.

Second, and more important to Clegg's argument, is the point that the official net migration figure tells us little about illegal immigration. Illegal immigrants, by definition, aren't registered. Much of what Clegg is expected to discuss looks at recent government advances in securing the UK's borders against illegal immiration, such as splitting the UK Border Agency (UKBA) in two to create an arm of the agency dedicated to illegal immigration.

Politically, however, this criticism is even more significant. Clegg is expected to claim that the Conservative party have realised their error, withdrawn their commitment to the target, and the government has been more effective in controlling illegal immigration as a result. In the early years of this parliament, he argues, the government didn't focus on getting all entry and exit points to the country covered (65 per cent were dealt with by March last year). Following his intervention, he will say, 80 per cent of these points are covered. The net migration target has long been known to be a source of contention within the coalition (past critics of the target have included Liberal Democrat Business Secretary Vince Cable), but for the Deputy Prime Minister to stand up and say so is a bold move.

If things are this intense now, the arguments can only get fiercer as the long summer recess drags on.