What if, as current UK Prime Minister Theresa May recently did, Gordon Brown called a snap election in 2007 but this backfired on him? The conventional wisdom is that Brown should have called a snap election and would likely have won a workable majority. That is possible, but as the recent snap election showed, a snap election call could easily backfire. And in 2007 the polls were close between Labour and the Tories, though most showed Labour narrowly ahead, and therefore a hung parliament was a possibility. The immediate pre-campaigning also suggested that Cameron could stage a comeback, as the Blackpool conference went brilliantly for the Conservatives while Brown made a tactical blunder in his trip to Iraq. It is possible that in the actual campaign Brown's honeymoon would have worn off and it could have ended in a hung parliament. Here are the results I made for TTL 2007.
2007 UK election
Gordon Brown-Labour: 314-41 39.2%
David Cameron-Conservative: 295+97 41.6%
Menzies Campbell-LibDem: 10-52 11.5%
646 seats
324 for majority

What would be the effects of these alternate results? Would Brown be able to form a minority government or coalition with support from the Liberal Democrats and possibly the Nationalist parties to gain a bare majority? How would the Labour Party react to losing their majority? What would happen to Brown's leadership? What about the LibDems-note that under these results OTL 2007 leadership contender Chris Huhne loses his seat but Nick Clegg keeps his)? How long could Labour last under these conditions? How would UK politics be affected? What would be the effect on the upcoming financial crisis-would Brown or whoever is PM by that stage(if another election hasn't already occurred) be able to handle the crisis? What if?
 
What plausible reason is there for the collapse in the LibDem vote in 2007 you propose? They're not tainted by coalition government as in 2015 and they're riding high on opposition to the Iraq invasion, at worst they're probably looking at retaining the vast majority of the 52 seats they had in 2005. A tactical blunder in campaigning from Brown's Labour isn't going to have LibDem voters flocking to support the Tories.

If it happens as you suggest, then yes, you likely end up with a coalition of Labour, LibDems and one or both of the SNP and Plaid Cymru as required.
 
IOTL under Mendes Campbell in 2007 the LibDems were polling around 12% and I put them on the low end of that due to their OTL 2005 and 2010 underperformance.
 
Even if they get 12%, they are not ending up with ten seats. The 2015 massacre was the unwinding of tactical voting - Labour supporters no longer voted Liberal to keep the Tories out. Here, that tactical voting still holds. At worst, I think they'd end up with 25-30.
 
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