New York City Mayor Michael Bloomberg (l.) and Rep. Joe Sestak (D) of Pennsylvania, take questions during a news conference in Philadelphia, Penn., on Aug. 17. Bloomberg endorsed Sestak in his bid to fill Sen. Arlen Specter's Senate seat. Matt Rourke/AP
Pennsylvania Republican Senate candidate Pat Toomey makes remarks at a news conference held by the Council for Citizens Against Government Waste in view of Independence Hall in Philadelphia, Penn., on July 19. Matt Rourke/AP
Rep. Joe Sestak speaks to reporters on Capitol Hill in Washington on May 28. Charles Dharapak/AP
Rep. Joseph Sestak is seen in his office on Capitol Hill in Washington on May 20. Sestak defeated Sen. Arlen Specter in the Democratic primaries and will be the Democratic candidate in the 2010 election for Specter's senate seat. Harry Hamburg/AP
On Jan. 30, Sen Arlen Specter speaks at a gathering sponsored by labor unions and citizen activist groups in Harrisburg, Pa. Carolyn Kaster/AP
Rep. Joe Sestak (l.) and Rep. Pat Toomey take part in a town hall meeting in Allentown, Penn., in September 2009. Rick Smith/AP
Veteran Republican Sen. Arlen Specter of Pennsylvania discusses his new Democratic Party allegiance at the White House with President Barack Obama on April 29, 2009. J. Scott Applewhite/AP
Britain's general elections used to be straightforward: a predominantly two-horse race between Labour or Conservative governments, left vs. right, red against blue. But as electioneering gets under way for May 7 polls, it's clear the vote has implications for British politics that extend well beyond whoever comes out on top.