The Conservative Party was re-elected last May promising to hold a referendum before 2017. Today, the battle over Britain's membership in the EU began a new phase after European Council President Donald Tusk published details of a deal with British Prime Minister David Cameron.

The deal allows for an "emergency brake" on in-work benefits for EU migrant workers. This would have to be agreed by other EU nations and it would be "graduated", with more money from tax credits paid to migrants the longer they remain in the UK. It also contains measures for the protection of countries outside the common currency. Cameron, whose goal was to negotiate changes to the relationship in order to keep Britain inside the union, said that the deal would deliver the "substantial change" he demanded.

"The proposed reforms still need to be adopted by the Council – and they are humble," says Rem Korteweg, senior researcher at the Center for European Reform. Most will not dramatically change the way in which the EU operates. There are some cosmetic changes which are very important to British politics – such as the proposal to exclude the UK from ‘ever closer union.'

There are two areas where changes are meaningful and controversial. The first is the emergency brake, which challenges the principle of freedom of movement and stretches the limits of current EU treaties. The second is a proposal to protect non-Eurozone countries from possible Eurozone caucusing.

Much remains unclear about this latter proposal. While meaningful, these two reforms are not likely to make a major difference to the average Briton: the emergency brake won't make a massive dent in the numbers of migrants coming to the UK, and the question on ‘Euro-Ins vs Outs' is important to London's financial center, but is very technical and won't persuade many Britons to vote in a particular way during the referendum.

Prime Minister Cameron held talks with EU President Donald Tusk

Dividing lines

Provided there are no major upsets at the summit taking place on February 18-19, the momentum will now shift from the halls of power in the EU to the domestic political arena in Britain. The referendum could take place as soon as June 23, meaning a short campaign. This is because Cameron is keen to hold the referendum before the summer gets underway. It is thought that the renewed migrant crisis over the summer months could make the public more likely to vote for an exit.

Cameron's strongest challengers on EU membership are within his own Conservative Party. Political analysts are watching to see which way members of the cabinet and backbench MPs will go. "Today's proposal sets out the contours of what the deal is going to look like," says Adam Hug, director of the Foreign Policy Center. "There will be a number of Euroskeptics who were willing to leave the door open [to remaining in the EU] if Cameron were able to get a concrete deal on migrant benefits. But this has a lot of caveats, which will nudge them into not accepting the deal. The summit may see further tinkering that may tip more out of the boat."

In or out? Polls show that Britons are fairly evenly divided on EU membership

Charm offensive

Ahead of the February summit, Cameron will be on a charm offensive across member states, trying to convince them of his plans. He is expected to face the biggest problems in Eastern Europe, where his proposals on restricting benefits are seen as discriminatory. "It is recognized that having Britain outside would be negative for the EU as a whole, but certainly also for Britain," says Fabian Zuleeg, chief executive of the European Policy Center. "There will be an effort made, but most member states see this very much as a process driven by the UK, so the UK has to put proposals on the table about how it can be done. What the UK cannot do is touch some of the fundamentals of how the EU works – the real treaty change procedure is the last thing anybody wants at this point."

Given that changes to the fundamental principles of the EU, such as freedom of movement, were not on the table, many believe that this deal is the best Cameron could get. However, concessions on issues such as the emergency brake leave the deal open to criticism from the anti-EU campaign. "It isn't 100 percent Cameron's negotiating position, which will cause him problems on the back benches," says Hug. "The fact that the EU wasn't able to respond absolutely to something that will be portrayed by Euroskeptics as a reasonable request will be played on by the ‘leave' campaign. But this was always likely to happen – whatever was brought back was not going to be enough for those people."

While politicians decide which side they will be campaigning on, it is also worth noting that the British public, by and large, has not been following the detail of the negotiations closely. Current polls show opinion split almost exactly down the middle, with a tiny majority in favor of staying in the EU. "In the end, it is still the choice of the British people to decide whether they want to be part of the EU or not," says Zuleeg. "This negotiation is not going to make that much difference to that."