The second MP was Alun Michael (Labour and Co-operative Party) who served 25 years in Parliament before announcing his resignation from the House of Commons on 22 October 2012. Michael's affiliation with the Co-operative Party did not appear on ballot papers in the 2010 election because the Electoral Commission ruled that any joint candidates who wanted the names of both their parties included on the ballot paper could not also display the Labour red rose logo. Michael opted to drop the reference to the Co-operative Party but after the election denounced the ruling as "an outrageous piece of incompetence by the Electoral Commission".
In 2012 Alun Michael was selected by the Labour and Co-operative Parties as their candidate for the election of a Police and Crime Commissioner for the South Wales Police force area and announced he would be standing down from Parliament. He accepted office as Crown Steward and Bailiff of the Chiltern Hundreds on 22 October 2012, thus vacating his seat. Police and Crime Commissioner nominations closed on 19 October 2012 at which point Michael's PCC candidature became official. A writ for a parliamentary by-election in Cardiff South and Penarth was moved on 23 October 2012 for a by-election to be held on 15 November 2012.
Cardiff South and Penarth was viewed as one of the safest Labour seats in Britain but ever since 1997 Alun Michael had seen his majority being progressively eroded. The 2010 general election had continued this trend, with the BBC reporting Labour's vote(17,262) declining by 7.7%, Conservative support (12,553 votes) rising by +4.4%, and Liberal Democrat votes (9,875) rising by +2.4%.
However, in by-election held on 15 November 2012, Labour's decline was reversed - although on a basis of very low turn-out (down 38.2% on the 2010 General Election). Labour's Stephen Doughty succeeded Alun Michael with 9,193 votes, comprising 47.3% of the overall vote. This was an increase (in share-of-the-vote terms) of 8.4% on Michael's 2010 performance. However, in terms of actual votes cast (9,193 compared with 17,262 in 2010), it was Labour's lowest-ever vote in this constituency. Doughty thus became the third MP in the constituency's history.
In the 2012 by-election Conservatives saw their support slide by the identical percentage by which Labour's rose (8.4%) and achieved only 19.9% of the vote (3,859 votes). The Liberal Democrats' support declined by even more than that of their UK Coalition partners (down 11.5%). Plaid Cymru's share of the vote rose to 9.5% (garnering 1,854 votes, up 5.4%). Also performing well in percentage terms were the UKIP with their support rising by 3.1% (to 6.1% of the total). Greens also increased their share of the vote to 4.1% (a rise of 2.9%). Veteran Communist campaigner Robert Griffiths saw his support rise slightly to a 1.1% share of the vote.
1983-2010: The City of Cardiff wards of Butetown, Grangetown, Llanrumney, Rumney, Splott, and Trowbridge; and the Borough of Vale of Glamorgan wards of Alexandra, Cornerswell, Llandough, and Stanwell.
2010-present: The Cardiff electoral divisions of Butetown, Grangetown, Llanrumney, Rumney, Splott, and Trowbridge; and the Vale of Glamorgan electoral divisions of Cornerswell, Llandough, Plymouth, St Augustine's, Stanwell, and Sully.