David Cameron has said the Conservatives and the Lib Dems share a number of policy aims. Gordon Brown has said he is also ready to talk to the Lib Dems, pointing to "substantial common ground". So how do the three parties' key policies compare?
ON TAXES AND SPENDING
The Conservatives want to make an extra £6bn of efficiency savings in the current financial year. They would reverse Labour's planned 1% rise in National Insurance (NI), which they describe as a "tax on jobs". They want to raise the inheritance tax threshold to £1m and incorporate an allowance to recognise marriage and civil partnerships in the taxation system. The Tories also want to impose a public sector pay freeze.
Labour say to cut government spending before the country is well clear of recession could put the economic recovery at risk and would not begin to reduce the budget deficit until next April. They say the NI rise is necessary to maintain investment in public services. However, they say they would halve the annual spending deficit by 2014 through cuts to "lower-priority" spending.
The Lib Dems are opposed to reducing government spending and propose to delay this until 2011-12. They would not scrap the NI increase, although their manifesto recognises it as "a damaging tax on jobs and an unfair tax on employees" and would try to reverse it "when resources allow". They campaigned on making income tax "fairer" by allowing people to earn £10,000 tax-free, funded by a "mansion tax". They want to cap public sector pay rises at £400. Their plans for a local income tax to replace council tax are rejected by the other parties.
ON ELECTORAL REFORM
A shake-up of the
is likely to be the Lib Dems' key bargaining tool in return for their support.
The Conservatives are opposed to reform of the current first-past-the-post system and have previously rejected the prospect of proportional representation (PR). However, Mr Cameron has talked of "compromise", including the promise of a committee of inquiry on the subject. His party's plans for reform include reducing the number of MPs to 585, cutting their pay by 5% and making constituencies the same size.
Labour wants a referendum on the alternative vote system, to introduce fixed-term parliaments and to reduce the voting age to 16.
The Lib Dems want to the single transferrable vote system of PR, to cut the number of MPs by 150 and to introduce fixed-term parliaments. Mr Clegg has previously said that changing the voting system was a prerequisite for his party's support.
Mr Cameron has said the Conservatives support the idea of a "pupil premium" which gives schools in England extra funding to take on children from disadvantaged backgrounds. They also want to make it easier for parents to save closure-threatened schools and set up their own schools, and allow state schools to use international exams. They want to provide an extra 10,000 English university places.
Labour also plan a local pupil premium, along with one-to-one and small-group tuition for every primary school child falling behind, personal tutors and choice of "good" qualifications for secondary age children. They want to guarantee every young person education or training until 18, with 75% going on to higher education or workplace training by the age of 30.
The Lib Dems want a pupil premium, specifying a sum of £2.5bn to be given to head teachers to allow the average primary school class size in England to be cut to 20. They want to replace the national curriculum with a "minimum curriculum entitlement" in state-funded schools, with an independent authority brought in to oversee exam standards. Mr Clegg's party aim to scrap tuition fees within six years and scrap the target of 50% of people going to university.
ON ENVIRONMENTAL ISSUES
The Conservatives say they have a vision of a "low-carbon economy", reducing carbon emissions by 80% by 2050, will block plans for a third runway at London's Heathrow Airport and instead develop a high-speed rail line to connect the capital with Heathrow, as well as the cities of Birmingham, Manchester and Leeds. They also support new nuclear power stations and carbon capture to make coal a "low-carbon" fuel.
Labour want to increase the UK's target of reducing carbon emissions by 34% by 2020, with 40% of electricity coming from "renewables, nuclear and clean fossil fuels" by then. They support Heathrow's third runway, along with high-speed rail linking London, Birmingham, the East Midlands, Manchester, northern England and Scotland.
The Lib Dems want to create a "zero-carbon" Britain by 2050, block new coal-fired power stations, unless accompanied by highest level of carbon capture, and reject new generation of nuclear power. They ban airport expansion in the South East - including at Heathrow - and cut the roads budget to reopen closed railway lines, rather than directly creating a new high-speed line.
IN OTHER AREAS
The Conservatives oppose ID cards and would give innocent people the right to have their DNA removed from the national database. They back the continued use of the Trident submarine nuclear deterrent. They have pledged never to join the euro.
Labour want to press on with their ID card scheme and maintain the current rules on DNA data storage. They too would fund a replacement for Trident. They do not rule out joining the euro but promise a referendum on the matter.
The Lib Dems oppose ID cards and would remove the genetic fingerprints of innocent people from the DNA database. They say Britain should look at alternatives to Trident, with the support of European allies. The party is supportive of joining the euro but has promised a referendum.