Complex sentence: In 1999 Livingstone sought Labour's nomination to be the first elected Mayor of London; opposed by Labour Prime Minister Tony Blair, he successfully contested the 2000 election as an independent candidate, which led to his expulsion from Labour.
Split into simpler sentences: In 1999 Livingstone sought Labour's nomination to be the first elected Mayor of London, but was prevented by Labour Prime Minister Tony Blair. Instead, he successfully contested the 2000 election as an independent candidate, resulting in his expulsion from Labour.