Speculation is raging over who Barack Obama will choose to be his running mate. An announcement could come by week's end, ahead of the Democratic convention, which opens on Monday.

The list of potential vice-presidential candidates is widely believed to be down to four: Virginia Governor Tim Kaine, Delaware Senator Joe Biden, Indiana Senator Evan Bayh and Kansas Governor Kathleen Sebelius. Hillary Rodham Clinton is seen by some Democrats as a longshot.

Only Obama, his wife, Michelle, a handful of his most senior advisers and his two-member search committee know for certain who has been vetted and discussed.

Obama plans a typical day of campaigning today. His schedule is open after he concludes a bus tour of North Carolina and Virginia tomorrow.

A similar decision looms for Republican candidate John McCain.

His top contenders are said to include Minnesota Governor Tim Pawlenty and former Massachusetts Governor Mitt Romney.