Labour's leader Ed Miliband must be feeling pleased with himself right now. Labour are 9 points ahead in opinion polls and would certainly win an election held tomorrow, and the Coalition partners are barely on speaking terms.
But this does not mean Labour will definitely win in 2015. A lot could happen between now and then:
The economy could show signs of recovery. It wouldn't have to be a full recovery, just signs, because the Tories don't have to convince the nation that they are competent at running the economy, merely that they are more competent than Labour, who are still distrusted. Probability: medium.
Argentina might invade the Falklands, Britain might win the war, and Cameron be re-elected in a surge of partiotism. Probability: low; they lost last time, and are unlikely to look for a repeat.
Boris Johnson might replace Cameron as Tory leader with enough time to make his mark before the election, he might re-inflate the Tory vote. If Clegg is also replaced, the Lib Dem vote might also go up. And if the two new coalition leaders visibly get on, who knows what might happen? Probability: low, even if Boris fantasizes about this.
The Tories plan to change how people register for elections. This might lead to would-be Labour supporters being less likely to register, boosting the Tories. Probability: medium; gaining an electoral advantage is probably the main reason the Tories are doing this.
The Eurozone might collapse. The Tories might respond with anti-European little Englanderism, winning popular support. Probability: high; it would surprise no-one if the Eurozone went tits-up.
Scotland might vote for independence, costing Labour 40+ seats. Probability: medium.
Labour shadow ministers might make serious gaffes, causing them to lose support. Probability: low; as a general rule, governments lose elections rather than oppositions win them.
Something else I haven't thought of might make a Labour victory less likely. Probability: medium; unforseen events are bound to happen between now and 2015.
For all these reasons, a Labour victory isn't guaranteed. But Labour might be able to govern before the next election: Miliband could offer the Lib Dems a deal of Lords reform in return for confidence and supply. If the Lib dems have any sense they'll take it (it's more than the Tories will give them). If Miliband could gain the support of the SNP as well (perhaps offering them a second question on the independence referendum), he's home and dry.
The mechanics of how it would work is that there would be a vote of no confidence in the government, which Cameron would lose. Then Miliband would a have 14 days in which to assemble a Commons majority for a Labour government, and win a vote of confidence.
From Labour's point of view, this would make a lot of sense. They believe that they would govern better and more fairly than the Tories, so a Labour government would be in the national interest. Furthermore, the current coalition, with two parties at loggerheads, doesn't make for effective government.
And by being in government Labour would be more likely to win in 2015, provided they don't cock it up. The big issue right now is the economy, and since it's running way under capacity, all they have to do is institute a big spending programme, paid for by printing money. (This new quantitative easing is unlikely to cause inflation, which is lower now than before the present QE of £375 billion was done.) This could fund infrastructure projects such as railways, house building, fast broadband, etc. They could also directly fund small businesses, rather than going through the banks as the Tories are doing.
Labour could also make sure that electoral changes won't hurt their support. They could, for example, make electoral registration a requirement of claiming benefits or studying at university. And if Labour nevertheless look likely to lose in 2015 they could always change the electoral system to STV at the last minute, guaranteeing the Tories will never rule alone again (and the Lib Dems aren't likely to want to deal with them again any time soon).
Labour could also ban large donations to political parties (hurting the Tories), and call for prosecutions where there is apparent corruption. For example, Health Secretary Andrew Lansley wants to part-privatise the NHS and has received bribes donations from private healthcare providers.
There will be 3 by-elections on the 15th of November, in Manchester, Corby, and Cardiff. If Labour do well in those elections, might that be a good time for them to make overtures to the Lib Dems? The worst that could happen is they'd say no, in which case Labour would lose nothing.