More than anyone, Brown, 50, is the politician who has made Labour acceptable to business and the middle classes. He has surprised many critics by running a budget surplus in Labour's early years. Now he faces the tougher task of delivering the improved public services that voters want without wrecking public finances. His vote on whether Britain joins Europe's single currency will also be key.
Brown and Blair have been close collaborators and rivals since they shared an office together after first being elected to Parliament in 1983. They are credited with the reforms that brought Labour from the wilderness of unelectability for 18 years to the 1997 landslide. Yet for all the accomplishments, their partnership is tense.
It wasn't very long ago that British political circles recognized Brown as the more seasoned of the duo. In the 1970s, Brown was well known as a long-haired student activist, and later in Scottish Labour circles. Blair, now 48, aspired to rock stardom and flirted with the priesthood before turning to a political career.
But when Labour's leader, John Smith, died suddenly of a heart attack in 1994, Blair prevailed in a power grab that still rankles in the Brown camp. Brown's supporters say his price for standing aside for the more telegenic Blair was a promise of control over economic policy when Labour came back into power. That is certainly the way things have played out.
Brown's big ego has led to plenty of friction between his aides and Blair's team at 10 Downing St. The Treasury has behaved as though it were an independent power center that Blair has been unwilling or unable to rein in. How well Brown and Blair work together will be crucial in their second term--assuming Labour, as expected, wins the election on June 7.