"You know one of his favorite phrases is that I would be a Bush third term. Well I think maybe his proposals could be Carter second term," McCain told Fox.
If we assume that McCain is being clever, it's a nice stilleto stab at Obama's relative weakness with Jewish voters. Jimmy Carter's been working these last 4 years (and arguably longer) to become a Goldstein figure to AIPAC-style Jewish voters, with the publication of Peace Not Apartheid, with his meetings with Hamas, and with slashing rhetoric about the Zionist state.
But if we don't assume that, what is McCain trying to do? The Obama-Carter comparison has been bubbling up on the right, sometimes to talk about Obama's personality, sometimes to talk about the political coalition he could build (for the right) if he failed like Carter, sometimes to talk about energy. But the salience to voters… well, is there any? Reagan defeated Carter when Barack Obama was 19 years old. The Carter presidency predates MS-DOS, VHS tapes, and Cabbage Patch kids. When the Simpsons joked that Carter was "history's greatest monster" back in 1993, and the people of Springfield tore down his statue, it was already ironic.
On second thought, this is an even worse argument for McCain than I thought: He's not rebutting the idea that he's going to continue unpopular Bush policies. It's almost as if the McCain campaign is an echo chamber of well-heeled Republican consultants with very few new ideas. I know, it sounds ridiculous.
UPDATE: To respond to the comments… of course McCain is trying to link Obama to bad memories of the Carter years. The problem is that Carter, like every ex-president, has improved his image over time, and the historical revisionism about his term has been going on for about a decade now. Yes, there are voters who remember every painful second of the Carter years and became staunch Alex Keaton-ish Republicans because of it. But stack them next to the 100 percent of the electorate that's living through the Bush II years. One argument connects with political junkies and voters who remember their history, and one argument connects with everybody. (The Carter revisionism has certainly been sped along by Bush's failures. It's harder to keep raging about the gas shortage when, in the here and now, you're paying $4 a gallon for gas.)