David Cameron and Nick Clegg are not one-man bands. They need to win over their parties if they are to press ahead with any deal. Here is a breakdown of some of the issues they would have to overcome.

The Liberal Democrats pride themselves on their party's internal democratic safeguards. They have what they call a "triple lock" on anything that could impact on their political independence.

This means leader Nick Clegg has to get the backing of 75% his MPs, and 75% of his party's federal executive, which includes elected grassroots members, for any deal with another party.

If he failed to get sufficient support he would have to call a special conference - probably within a week - which would have to approve any deal by a two-thirds majority. If that failed, he would have to consult all members in a postal ballot and gain a majority of support before pressing ahead.

Ideologically, many Liberal Democrats feel closer to Labour than the Conservatives and will be unhappy if Mr Clegg does a deal with Mr Cameron. Senior Lib Dem Simon Hughes told the BBC the parties were not "immediate natural allies".

Even among the negotiating teams trying to hammer out a deal in the Cabinet Office, the fault lines are obvious. Danny Alexander, Nick Clegg's chief of staff, is a former leading light in the campaign for Britain's entry into the euro, while William Hague fought a general election campaign as Tory leader on a "keep the pound" platform.

There has long been talk of a "progressive alliance" between the Lib Dems and Labour, to keep the Tories out of power, although some of the new breed of free market Lib Dems around Nick Clegg do not view themselves as being naturally left wing, in the same way that much of the party's grassroots do.

There are several examples around the country of Conservatives working with Liberal Democrats at a town hall level, but the supporters of the two parties are miles apart on many issues and often view each other with suspicion or downright hostility.

Former Lib Dem MP Evan Harris, who remains an influential voice on the left of the party, told the BBC Mr Clegg would be "under huge pressure from, I suspect, the party membership to go nowhere near the Conservatives".

And some Conservatives would rather see Mr Cameron attempt to form a minority government than do a deal with their political opponents. Former party chairman Lord Tebbit said that would be the "appropriate thing" and told the BBC that, among wider Conservative membership: "I think there's no mood in the Conservative Party to bring the Liberal Democrats into government."

The two parties are diametrically opposed on the issue of voting reform. Scrapping what they see as Britain's unfair first-past-the-post system in favour of proportional representation has been a key Liberal demand for decades and the traditional "deal breaker" in previous coalition negotiations. The Conservatives are wedded to first-past-the-post, arguing that it delivers strong government and allows voters to kick out unpopular governments.

As the first step towards a compromise, Mr Cameron has offered an "all-party committee of inquiry on political and electoral reform" - something that is unlikely to satisfy many Lib Dems. He has also proposed equal-sized constituencies and fixed-term parliaments.

But the Tory leader may find it difficult to go much further, as many Tories fear a move to PR would lock them out of power forever.

Fraser Nelson, editor of the Conservative-supporting Spectator, wrote on Friday: "Inquiry into electoral reform? The Spectator cannot support this."

But former Conservative minister Michael Portillo told the BBC he did not think Mr Cameron, a moderniser, had "set his face against changing the electoral system".

But Labour's pledge of a referendum on electoral reform may not be enough for the Lib Dems either. During the campaign Mr Clegg has dismissed the idea of a referendum on switching to the Alternative Vote system as "a baby step in the right direction". He is also thought to fear that the party could be punished in any referendum on voting reform if they strike a deal with Gordon Brown.

Nick Clegg has been careful to refer to political, rather than voting, reform in his public statements on the coalition talks, suggesting PR might not be the deal breaker it once was.

And although the two parties share many policies when it comes to cleaning up politics, such as curbs on lobbyists, recall ballots for MPs, caps on party donations and an end to "non-dom" peers, there are some fundamental differences.

The Lib Dems want a written constitution, a fully elected House of Lords, as well as proportional representation for Westminster elections - neither of which are on the Tory agenda.

Mr Cameron favours a partly-elected Upper House, but it is not a priority for him. Former Lib MP Evan Harris said it would be "extremely difficult" to see how a deal with the Conservatives would be sellable "if there's nothing, not just on electoral reform, but on cleaning up politics on a wider scale".

Apart from electoral reform, Europe is probably the issue on which the two parties are farthest apart. The Conservatives want a Sovereignty Bill to halt moves towards further integration and to repatriate some powers from the EU. The Lib Dems are the most pro-European party in Britain who, until recently, were arguing in favour of joining the euro - something the Tories have said will never happen on their watch. They may be able to paper over their differences in the short term but it is bound to become a major stumbling block at some point.

On the face of it, immigration is another area where there are major differences between the Tories and the Lib Dems. The Conservatives want an annual limit on non-EU economic migrants, a new border police force and a tougher visa regime.

The Lib Dems favour earned citizenship for illegal immigrants, attacked by Mr Cameron as an "amnesty", and a regions-based migration system, also ridiculed by the Tory leader during the election campaign. The parties agree on a border force, however, and both would retain the current "points-based" migration system.

Both agree on the urgent need to reduce Britain's budget deficit and reform the tax system, but there are significant differences in how they would go about it.

The Tories would scrap Labour's National Insurance increase - something the Lib Dems could potentially sign up to. But there appears to be less Lib Dem enthusiasm for scrapping the 50% top rate of income tax. But the Lib Dems may be under pressure to ditch their complex plan to cut taxes for low earners, paid for by a "mansion tax".

Tory commitments on inheritance tax, stamp duty and marriage would not be a stumbling block in the short term, as they have only promised to introduce these once they have got the deficit under control.

The Conservatives are committed to replacing Britain's Trident nuclear missile system. The Lib Dems have ruled out a "like for like" replacement, arguing that cheaper options should be explored in the upcoming strategic defence review. Ed Llewellyn, David Cameron's chief of staff and former adviser to Paddy Ashdown, may be critical to finding agreement on this and other foreign and defence policy issues.

Another area with major differences between the two parties. The Lib Dems would scrap the government's prison-building programme and have fewer six-month sentences. The Tories want to build more prisons and end the early release scheme for prisoners. Both favour elected police chiefs in some form, however.

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