Nick Clegg is deciding whether to back a Conservative minority government or one of Labour and the smaller parties - he would get a better deal from the latter
The Conservative party are not offering the Lib Dems any solid pledge on a referendum on Proportional Representation. Cameron offering an all party commission on electoral reform does not specify who would appoint the members, nor give any guarantee that commission would recommend a referendum on PR, nor any guarantee that Cameron would accept it’s recommendations if it did
A minority government or coalition of all parties other than the Conservatives would represent around the same percentage of the electorate – at over 60%, that a Liberal-Conservative one would at 59.1%. The Conservatives got just 36.1% of the vote, merely the largest minority, so they have no unique right to form a government.
First past the post results in the votes of millions being binned unrepresented if they don't vote for the party that got the majority (or more often the largest minority) of the votes in that constituency. It also distorts how people vote as a result by encouraging them to vote for a party they see as bad in order to keep out one they see as worse. It results in 'safe' seats which result in big parties looking after the interests of big donors to party funds rather than those of voters.
P.R on the single transferrable vote or the additional member systems systems (the latter being the one used in Scottish Parliament elections) would allow people to vote positively for the candidate or party they agreed with most and would ensure that everyone's vote counted equally and was represented.
We need to scrap the backwards, undemocratic and archaic first past the post system now - and replace it with PR (STV) or PR (AMS).
A coalition or minority government of all parties except the Tories might well be unstable and find it difficult to agree on anything except a bill on PR and not making big public spending cuts until the economy recovers, but those two issues would be plenty – and they would disagree on them far less than Liberals and Conservatives would be likely to. Labour, with less seats, are also more likely to offer a solid pledge on a referendum on P.R than the Conservatives are.
Brown may need to pledge to step down as Labour leader within the next few months to get that deal. By doing so he would be remembered as a Prime Minister who put democracy and the good of both his party and the country before his own career. He might well even be made a Minister in the new government.
Clegg only needs a coalition or minority government which includes a pledge on a referendum on PR within months. It doesn’t need to survive long to do that – and then the next election would be under P.R. - with very different results, whether thye next election is months or years away.
If Clegg accepted a deal that does not deliver full PR (which excludes the 'Alternative Vote'), whether from the Conservatives or Labour, his party will split and most Lib Dem voters will abandon his party entirely.
Even if propping up the Conservatives or Labour lost the Lib Dems half the votes they got in the last election at the next one, they would still increase their number of seats under PR. Currently they have under 10% of the seats in parliament (57 seats) from 23% of the vote. Even if their vote halved in a future PR election (and it would be unlikely to as under PR more people would feel they could vote Lib Dem without letting Labour or the Conservatives in) they would get 12.5% of the seats, or 81 seats - a big increase.
Supporting any government without pledges on PR and delaying spending cuts would put the Lib Dems in the same position they were in in the Scottish Parliament in the past - as the junior partner in a coalition with little influence on most of government policy, but being held responsible for all government policy by the voters - and losing seats in future as a result.