Labour has lost both by-elections to independents in the south Wales constituency of Blaenau Gwent.
Labour conceded long before the results in the Welsh Assembly and Westminster seats were declared early on Friday.
The death of independent AM and MP Peter Law caused the elections, and his widow Trish became AM with a 4,400-plus majority. Fellow independent Dai Davies became MP with a near-2,500 majority.
First Minister Rhodri Morgan called it a "massive disappointment".
Bur he added: "It doesn't make things worse than they've been since May of last year.
"We've got used to governing with a majority of minus one, shall we put it that way. And we've got another 10 months to go before the assembly elections on 7 May."
Mrs Law said: "This is a victory for Blaenau Gwent: a victory for honesty and integrity".
She promised to be a "voice for our communities" and to "ensure that never again will the area be taken for granted".
Dai Davies, ind: 12,543 (- 7,962 from 2005)
Owen Smith, Labour: 10,059 (- 1,325)
Steffan Lewis, Plaid Cymru: 1,755 (+ 912)
Amy Kitcher, Liberal Democrat: 1,477 (-34)
Margrit Williams, Conservative: 1,013 (+197)
Alan Hope, Official Monster Raving Loony Party: 318
Labour had been desperate to win the assembly seat because it would have given the party a working majority in Cardiff Bay. It lost that majority when Mr Law became an independent to fight the 2005 Westminster election.
Before Mr Law's victory last year, Blaenau Gwent had been Labour's safest parliamentary seat in Wales, and the fifth safest in the UK.
In his acceptance speech, Mr Davies said: "A political revolution is starting in Blaenau Gwent tonight.
"Political parties take note. You take people for granted at your peril. It's the people that matter, not the political parties.
"The dinosaurs thought they would live for ever - they died out. Political parties take note and listen to the people or you're in trouble."
Two lots of Blaenau Gwent votes were counted in Ebbw Vale
As Labour candidates Owen Smith and John Hopkins arrived at the count in Ebbw Vale, they were taunted by independent supporters, who chanted: "Lost again, lost again".
Mr Smith blamed Labour's defeat in what used to be its heartland seats on the circumstances of what he called a "unique by-election".
He said: "A huge number of people did put their faith back in Labour, and we are very grateful to them for that, but quite clearly it wasn't enough."
Labour MP Wayne David said his party had "a lot of listening to do".
Party chairman Hazel Blears said: "Clearly there are issues here for us and we're listening, taking that seriously".
Trish Law, ind: 13,785
John Hopkins, Lab: 9,321
Steve Bard, Lib Dem:2,054
John Price, Plaid: 1,109
John Burns, Con:816
John Matthews, Green: 302
Mr Law became the MP in 2005 when he left Labour to fight the Westminster election as an independent in protest at Labour's use of an all-women shortlist to select the parliamentary candidate.
Iconic Labour figures such as Aneurin Bevan and Michael Foot used to be the MP for the area, and before the 2005 election the party held a 19,000 majority.
But Mr Law turned that into his own 9,000 majority. He died in April of a brain tumour, less than a year after becoming the MP.
There was a high turnout of 51.7% for the parliamentary ballot and 52.1% for the assembly vote.
At the last assembly election in 2003, turnout was 37.4%, which was slightly lower than the all-Wales figure of 38.2%.
The double by-election with two polls in the same constituency was an unprecedented situation in Wales.