On the left we can see Mitt Romney, and on the right Rick Santorum, the two front-runners in the race for the Republican presidential nomination (their names have been changed for comic effect). The woman in the middle is Barbara Bush, wife of 41st U.S. president George H.W. Bush, and mother of the 43rd president George W. Bush, whom we can see in the foreground. The former first lady went on the airwaves on Monday to complain about the nastiness of the current presidential race, calling it "the worst campaign I've ever seen in my life". In the cartoon, she comments: "Worst race ever! Makes my boy look like ..."
The joke is that George W. (aka Dubya) is dressed up as first U.S. president George Washington. If Romney and Santorum make George W. look like Washington, then things are looking really bad for the Republican party! (The fact that current favourite Romney lost out to John McCain in the race for the 2008 Republican nomination, while McCain was defeated by George W. Bush in 2000 says it all—the most likely Republican candidate is the man who lost to the man who lost to Dubya—widely regarded as one of the worst U.S. presidents ever!)
Steve Bell regularly drew George W. Bush as an ape, hence the ears and the tail.
Worst is the superlative form of bad. See here for a discussion of the difference between 'worse' and 'worst'.
• Super Tuesday: Romney wins Ohio but Santorum keeps race alive (The Guardian)
• Steve Bell on George Bush (The Guardian)
• Clay Bennett cartoon (Chattanooga Times Free Press)
• Barbara Bush, Hater of ‘Ugly’ Campaigns? (ABC News)
• Not so Super Tuesday. But is it worst ever in Republican nomination race? (The Guardian)