
How current UKIP supporters said they voted at the general election
The threat to Labour
1. An EU referendum on election day in May 2015 would turn out the Tory vote in huge numbers.
Entirely possible, assuming that a referendum is held on election day. A referendum could be a useful tool for the Tories in Labour/Tory marginals to ensure high Conservative turnout, and it could only be helpful in the South West Conservative/Lib Dem marginals. Conservative voters would be more enthusiastic about going to the polls in favour of a Party with a strong message on the EU and the economy, in contrast to Labour’s perceived weakness in those areas. A UKIP that is able to maintain it’s relevance in the event of a referendum could scrape off weak Labour support closer to the election (the above graph shows that to be an unlikely outcome as things stand)
2. A distraction
Assuming that the economy does not improve, Labour’s improving reputation on economic trustworthiness would be less useful if the campaign were fought over the EU. Labour voters are (as things stand) anti-EU (see below graph), and if Labour were to take a pro EU stance in any referendum, then our own vote could be weakened; post 2010 switchers could be encouraged to defect back to the Conservatives, and hurt the significant progress that has been made since 2010. 
The threat to the Tories
1. A distraction
It is very unlikely that a referendum would be held on the same day as the next General Election the 2015 election would be turned into a single issue campaign that would only hurt the Conservatives. Not only would they be forced to take a party line on the in/out question (which would lead to an inevitable internal bloodletting) but they would be running a campaign on Europe, not the economy, which is their best chance of winning a general election, assuming that they economy improves slightly.
2. Hurting the Tory vote in marginal constituencies.
Interestingly, an EU referendum could actually hurt the Tories most in two key areas: the Midlands and London, where the Conservatives need to win a combined additional 30 seats to claim a Parliamentary majority. London is the only region in favour of staying in the EU at the moment, favouring staying in by 41 to 40%, whilst the Midlands has the 2nd narrowest gap in favour of leaving by 34 to 49% by contrast, the South of England supports leaving the EU by a margin of 19% – 49 to 30% – meaning that they would be racking up votes in their already safe constituencies in leafy Hampshire and Surrey.
3. Party infighting.
Since becoming Prime Minister, David Cameron’s biggest threat to his leadership has been over the EU. The largest Conservative Parliamentary Party rebellions have been over the EU, both on a referendum (81 MPs) and on the size of the UK’s contribution to the EU (53 MPs). Although perhaps not as serious a threat to the party leadership as under John Major, the pro-EU group featuring Ken Clarke and Michael Heseltine are still important figures in the Conservative Party and the Coalition Government. A majority of Conservative MPs are not yet better off outers, but that point becomes closer ever week.
4. UKIP
18% of UKIP voters would consider voting Tory. 31% of Lib voters would. Perhaps an indication of where the Tories should be looking for new votes from one of Britain’s Third Parties?
Michael Fabricant’s intervention today, where he argued that an electoral pact with UKIP would:
a) have taken the first step in being in tune with popular opinion;
b) have neutralised a UKIP threat; and
c) attenuated the politicking of the Labour party on the European Question
This proposal has been strongly rebuffed by Nigel Farage who has
declared “war” on the Conservatives. Nice one, Mr Fabricant. In one great gesture you have single handily made UKIP more important, whilst getting rebuffed by them, all whilst making your own Party look weak.
Tim Montgomerie makes a persuasive case for the Tories in “dealing” with the UKIP threat:
If David Cameron doesn’t address Europe then the party will remain split on the issue and sections of our supporters will remain obsessed by it. If he does address the issue he has a chance of securing his base amongst centre right, Eurosceptic voters and can then focus on reaching out to floating voters who are most interested in bread and butter issues like the cost of living and the NHS.In Lord Ashcroft’s mega poll, it was found that of those who voted Tory in 2010, an equal proportion intend to vote for UKIP as intend to vote for Labour/LDs combined.

If the Tories formed an electoral pact with UKIP, they would benefit by winning roughly 20 additional seats. Just as a side note, if they went into a pact with the BNP they could gain between 11-14. Incidentally, the likelihood of either of those things happening is pretty much equal.
5. Becoming increasingly irrelevant
Tim Montgomerie (quoted above) argues that the Prime Minister should: “secure his base amongst centre right, Eurosceptic voters and can then focus on reaching out to floating voters who are most interested in bread and butter issues”. I suspect this is exposing Montgomerie’s own attitudes and beliefs on a referendum. If the Tories become increasingly focused on the EU and a referendum, then they will not be given an opportunity to address their already weak position on issues such as the NHS. The latest Ipsos-Mori poll puts Britain’s membership of the European Union as the top concern of just 3% of voters.