Jim Callaghan once described the idea of an EU referendum as “a little rubber life raft into which the whole party may one day have to climb”.

Judging by the initially sharply divided Conservative response, the little rubber life raft constructed by David Cameron and Donald Tusk, the EU council president, does not look very seaworthy.

The pessimistic mood among Tory Eurosceptics has visibly perked up, as for the first time they look at the deal designed to change the relationship between the UK and the EU, and frankly scoff, because Brussels has conceded precious little. .

Cameron can claim through an “emergency brake” he has fulfilled his manifesto promise to ensure EU migrants will not receive access to UK benefits for up to four years. But the commitment is heavily qualified. It is not known precisely how long the ban on benefits would survive, and the future criteria that would allow the UK to put on the brake is extremely imprecise and not quantified.

The ban is also not complete. It is graduated from “an initial complete exclusion but gradually increasing access to such benefits to take account of the growing connection of the worker with the labour market”. That suggests workers will start to receive some tax credits well within four years.

The text also makes clear the member state has to apply to the commission and to other EU council members to apply the emergency brake, and it is for the EU council, acting on a qualified majority vote, to decide how long the brake should apply.

But critically Cameron can point to the fact that the commission has agreed that current level of EU migrations into the UK warrants applying the brake. The UK government said the ban would take six months to be imposed after a UK application.

On this basis many other EU countries, notably Germany, could seek to apply the benefit brake, a point that is likely to unnerve the Polish and Bulgarian governments.

Iain Duncan Smith, the work and pensions secretary, will be a pivotal figure.He will for the moment keep quiet on the value of this deal – in line with the agreement to abide by collective cabinet responsibility until the final deal is agreed probably in Brussels on 18 February.

But in recent weeks he and his department have been consulted on the EU’s floated proposals as Downing Street picks them up from Brussels and Berlin, and has been left unimpressed. For a start, the deal does not stop any EU citizen coming to the UK, his friends point out.

Duncan Smith thinks if the UK minimum wage reaches the £9.40 an hour in 2020 – the level planned by the Treasury – migrants from eastern Europe will disregard any relatively small loss of tax credits and make the calculation that it is the level of wages, not benefits that makes the UK an attractive place to come. His ministerial deputy, Priti Patel, is likely to take a similar view, and the duo have access to a battery of DWP statistics, and apparently the support of the Office for Budget Responsibility, to make this point.

It will be interesting to see if Cameron or the home secretary, Theresa May, will specify how much they think this emergency brake will reduce the flow of EU migrants and whether the Conservative manifesto target of cutting net immigration to below 100,000 is now achievable.

In addition, Cameron has not got what he initially wanted on EU migrants sending child benefit to their children abroad. He wanted an outright ban but instead benefits will be index-linked to where the child is based.

Cameron has also not made huge progress on the relationship between euro ins and outs. There are various statements in the Tusk document asserting the rights of outs must be protected, including the right not to pay costs of a euro member states’ bailout, but the mechanism for protecting the rights of the euro-outs is left unclear.

Tusk, aware of French objections, says the objections of the outs cannot constitute a veto or impose unnecessary delays. This suggests the Treasury will get a three-month cooling off period which is unlikely to reassure the City of London that its key strategic interests have been definitely preserved.

The “red card”, trumpeted on Monday night by the hardworking Downing Street spin machine, is hardly a game changer. It requires 16 national parliaments to agree to it, and at the end the EU council is required to consider the objection. This is the equivalent of a football referee showing a player the red card, but it is for the player to decide whether his foul was so egregious he will choose to leave the pitch.

Moreover, the likelihood that the commission will table a directive that provokes more than half of the national parliaments of the EU to object is difficult to imagine. It is hardly an issue in the real world.

The other measures on exempting the UK from ever-closer union, or increasing competitiveness in the EU are largely rhetorical. Few commission officials believe they are working to undermine EU competitiveness.

But Cameron is the most gifted of salesmen. He will hope the debate will slowly shift from the entrails of treaty opt-outs and vetoes to the bigger choice of whether Britain wants to leave the EU or instead take the risk of a cold lonely life away from its main trading partner.

Cameron has won two electoral contests using fear: the Scottish referendum and the 2015 general election. There is no reason to think he will try something different this time.

He is clearly pushing for a June poll perhaps in the belief that the campaigns for Britain to leave the EU are in such internal disarray, and the Syrian refugee crisis so unpredictable, that it is best to act before the leave campaign gets its act together.

The rows between the two leave camps are often portrayed as being about personalities. But the mainstream leave campaign simply believes it cannot win on immigration alone, and believes its rivals are influenced by the belief that Ukip will benefit from a vote to stay in the EU.

The leave campaign knows it needs to take the risk out of a vote for Brexit. As a result it talks about the possibility of a second referendum once the final terms of a UK departure are negotiated. That means a first vote to leave the EU would be the equivalent of handing your negotiators their bargaining hand, but the issue of a final rupture could be reconsidered at a later date.

The leave campaign also knows it badly needs bigger names to support them than a relatively small group of Eurosceptic MPs and businessmen.

The leave campaign holds out the metaphor of nervous sceptics – in newspapers, business and politics – holding hands and taking the leap together. But Leave UK, has not given up the hope that one or two prominent political or media figures will look at this deal’s details, pluck up the courage and desert Cameron.

Looking at what has been negotiated in the past six months, it is still possible that one or two may yet take the plunge, so upsetting Cameron’s bobbing dinghy. The next two weeks will be critical not just for Cameron, but also for the Conservative party and Europe.