From FairVote.Org: Presidential Frontrunners Support Instant Runoff Voting:

The two frontrunners for their party's nominations, Republican John McCain and Democrat Barack Obama, are both active backers of instant runoff voting (IRV). In 2002, Sen. McCain recorded a message for backers of IRV in Alaska, while that year Sen. Obama was the lead sponsor of legislation to implement IRV for certain Illinois elections. With most third party candidates also supporting IRV, we may see a rare issue of consensus this November, although neither McCain nor Obama have yet secured their party's nomination.

• Senator John McCain's message on IRV
• Sen. Barack Obama's state legislative proposal on IRV