Europe has long been the dividing line in the Conservative party. It helped bring down John Major after he signed the Maastricht Treaty.
Since the rise of the Eurosceptic UKIP Eurosceptic Tories have become increasingly at odds with the party's stance on Europe. The referendum is more about keeping the Tories together than what's right for Britain.
Now the infighting has become more public, and as the referendum draws closer it's getting more heated. Boris Johnson and David Cameron go back a long way, they were at Eton and Oxford together. Johnson always assumed he would be the more prominent politically as Cameron didn't get involved so much in politics while he was at Oxford while Johnson was very active.
The reverse happened, and the rivalry has not gone away. Johnson long considered himself heir apparent, but his siding with the Eurosceptics was seen as opportunistic and even treacherous by a lot of loyalist Tories.
Now it's got nasty and personal.
Europe doesn't affect the other parties to anywhere near the same extent. There are a few Eurosceptic Labour MPs but most of the party is pro Europe. The Liberals and SNP are both for staying in too.