The daughter of Labour MP Ernest Armstrong, she was shortlisted for the vacant Sedgefield constituency in 1983, only to lose out to Tony Blair. Four years later, at the 1987 general election, she was elected to her father's North West Durham seat on his retirement, increasing his majority by 3,806 to 10,162.
Armstrong was seen as a politician on the centre-right of the Labour Party, and was close politically to her near neighbour Tony Blair and the New Labour agenda. However, she is also a member of the Amicus trades union (formerly MSF), and her trades union links were useful when she helped to shore up support for the rewriting of Clause IV.
Armstrong also faced criticism after government defeats in the Commons over the length of time suspected terrorists could be detained without charge, and incitement to religious hatred provisions in the Prevention of Terrorism Act 2005. Press commentators speculated that in losing these votes through miscalculating government support, and in one instance letting the Prime Minister off the "Whip" because she believed the vote was won, Armstrong's position had become vulnerable. However the rumours that she would resign the post proved unfounded.
Afterwards Conservative leader David Cameron mocked Armstrong during an exchange with Tony Blair, saying “She must be the first Chief Whip in history to put the Prime Minister in the frame for losing a key vote — which is an interesting career move, to say the least.” This was the second time David Cameron had attacked her during Prime Ministers Questions; on his debut as Leader of the Opposition on 7 December 2005 she was singled out by Cameron when he said "That's the problem with these exchanges - the chief whip on the Labour side shouting like a child. Is she finished? Are you finished?
In 2006 Armstrong launched a petition on behalf of the Bethnal Green and Bow Labour Party against Respect MP George Galloway's participation in Channel 4's Celebrity Big Brother. She criticised Galloway for being paid as an MP during the time he was in the Big Brother house. Galloway responded by saying he planned to refund the taxpayer after his exit from the show as he would not know how much to refund until then.
Armstrong formally resigned from the government on 27 June 2007 when Tony Blair resigned as Prime Minister. On becoming Prime Minister, Gordon Brown announced Armstrong's appointment as Chair of a Parliamentary Labour Party Manifesto Committee drawing up policy ideas covering children.
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