Barack Obama will on Tuesday step up his challenge to Hillary Clinton, the Democratic party’s presidential frontrunner, by staging 18 simultaneous rallies across the US to mark the fifth anniversary of his opposition to the Iraq war.

Mr Obama’s renewed emphasis on Iraq, which will kick off a four-day “judgment and experience” tour of the state of Iowa, is designed to highlight Mrs Clinton’s refusal to apologise for her 2002 vote in favour of the war. It is also intended to counter Mrs Clinton’s contention that Mr Obama lacks the experience necessary to govern the US.