[cnn-photo-caption image= http://i.cdn.turner.com/cnn/2008/images/03/30/art.mackerry0330.gi.jpg caption="Sens. Kerry and McCain share an unusual history together in presidential politics."]
(CNN) - Stumping for Sen. Barack Obama, Sen. John Kerry faces a unique challenge when it comes to taking on the presumptive Republican nominee.
That's because Sen. John McCain has said that Kerry, in 2004, "asked if I would consider being his running mate." McCain told reporters earlier this month that at the time of the conversation, he made clear to Kerry the answer was no.
Kerry has said McCain's representatives contacted him about the possibility, and both men say there was never an actual offer.
McCain went on to campaign aggressively for President Bush in 2004.
Still, the complexity adds an unusual wrinkle for times like Sunday, when Kerry takes on McCain.
"Hillary Clinton has every right in the world to continue to fight, but the important thing is to be fighting against John McCain," Kerry told ABC's "This Week," discussing some calls for Sen. Clinton to drop out of the race.
"It is very important for both people to keep the eye on the real target - John McCain and the Republican disaster of the last seven and a half years," he added.
As the 2004 Democratic presidential nominee, Kerry is one of Obama's most prominent campaign surrogates.
–CNN's Josh Levs