With so much at stake, analysts said Clinton needed a very strong performance in the debate, the only time the two candidates will share a stage in Texas before the state's primary.

CNN: Obama, Clinton agree to disagree

Johnson did have his moment in the spotlight on TV primetime last year when sharing a stage with the likes of Romney in the Republican primary elections, he cracked a joke that made headlines.

BBC: US election: The third-party candidates

He stood up on the stage in one of his primary debates and proudly promised that his new tax cuts, on top of the Bush tax cuts, would include the top 1 percent.

WHITEHOUSE: Remarks by the President at a Campaign Event -- Miami, FL

But we recognise that his positions have evolved as the campaign has moved from the primary stage, where politicians have to outdo each other in their appeal to their party faithful, to the general election.

ECONOMIST: Barack Obama still has a lot to do

Barack Obama won Mississippi's primary and Wyoming's caucus by wide margins in the last stage of the Democratic presidential nomination process before Pennsylvanians vote on April 22nd.

ECONOMIST: Politics this week

It is not a technique in the primary science of Euclid, but Prof Nichol said it could be deployed at some stage.

BBC: Euclid telescope to probe dark universe

The tea party-backed Paul first stepped onto the national stage in 2010 when he vanquished McConnell's chosen Senate candidate in a GOP primary in Kentucky.

NPR: Senator Uses Old-Style Filibuster On CIA Nominee

After leaving office, Lamm abandoned politics to teach and write, returning to the stage for a brief and ignominious Senate race in 1992, in which he lost in the primary to Ben Nighthorse Campbell.

CNN: It's My Party And I'll Run If I Want To