To the Iraq War, you can add cosying-up to Big Business/the City, the acceptance that Thatcher wasn't all bad, ditching Clause 4, aligning with a neo-Con White House, and flipping ten-bob in the direction of pensioners, for evidence of how Labour's core vote started to dissipate.
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Their core Labour vote is roughly about what Ed Milliband reached at the last general election, 231 seats.
Off the top of my head I'm guessing this is what Gordon Brown achieved in 2010, probably other Labour leaders too. People shouldn't underestimate the fact that if you voted Labour in the 1980's to get rid of Thatcher, you've probably done so ever since out of loyalty, regardless of the Iraq War and other things you've mentioned.
The 1997 election, the 400+ seats... was unprecedented....
At this moment in time, (after 1997),- the issues Grasshopper mentions..... help to lose the winning vote, about 100-150 seats. (not necessarily the core vote).
There's a theory that Labours core vote went to UKIP last week because it was a UKIP issue, but it will come back to Labour again at an election. The problem is......at very best, this still gives Jeremy Corbyn...the same seat-share as other failed Labour leaders, i.e. the core vote, (about 250 seats).
He needs much more... like a 3 mile chaser, running at 2 miles in a novice hurdle. If everything falls in his favour, which it may not........... he still needs more...David Milliband in his cabinet, might help, but its unlikely.
Ps, checking results on the internet shows that since the 1974 election, the only leader to get in the magic 300's was Tony Blair. He has received way too much stick for a lot of things imho.
Everyone in parliament at that time is to blame for the Iraq war apart from Charles Kennedy and those that took a stand.