The most common comparison John McCain faces is, of course, to Still-President Bush. The problem for the Republican presidential hopeful is that the similarities are pretty overwhelming — they agree on practically everything; they’re both conservative Republicans; and neither seems to have any intellectual curiosity or tolerance for competing ideas.

The McCain campaign seems to believe this isn’t fair, not just because they don’t want to be tied to the least popular president in generations, but because they want a comparison for Barack Obama, too. Last week, McCain said Obama reminds him of William Jennings Bryan. Given that Bryan last ran for president literally 100 years ago, and most Americans only know Bryan for his role in the Scopes Monkey Trial if they know him at all, this was something of a dud.

So, McCain gave it another shot yesterday.

In separate interviews today, John McCain compared Barack Obama to Jimmy Carter.

Speaking to Fox’s Carl Cameron, McCain raised Carter’s name, putting a face on his assertion that Obama wants to return “to the failed policies of the ’60’s and ’70’s.”

And then, sitting down with NBC’s Brian Williams, he poured it on — and got to the heart of the matter.

“Senator Obama says that I’m running for a Bush’s third term,” McCain said, picking up the central Democratic line of attack. “Seems to me he’s running for Jimmy Carter’s second.”

I suppose it occurred to the McCain campaign to compare Obama to Bill Clinton, but given that the Big Dog left office with the highest approval ratings in a generation, that wouldn’t help. They could have gone with a JFK analogy, but that’s counterproductive, too. After ruling out other Democratic presidents like FDR, LBJ, Truman, and Wilson, McCain had a choice between Grover Cleveland and Carter. I suppose the latter has greater political significance right now.

But it’s hard to imagine this having much of an effect.

My friend Alex Koppelman explained McCain’s problem very well:

Given Bush’s historic unpopularity, the linkage between Bush and McCain is quite damaging for the presumptive Republican nominee. Moreover, Bush’s presidency is fresh in voters’ minds. On the other hand, Carter lost his bid for reelection to the presidency in 1980. That means anyone who was old enough to vote for or against him will be 46 or older by the time this election rolls around.

According to exit polls from 2004, at least 46 percent of the people who voted for president last time around were younger than that. That’s a whole lot of people who might not get the joke as fully as McCain would like.

Or, as Oliver Willis noted, “McCain went from a 19th century comparison to a 20th century comparison. At some point in his campaign I suppose he might join Sen. Obama and the rest of us in the 21st century.”

That would be nice. And if he does join us in the 21st century, maybe he can learn what Google is.

[Yesterday], Sen. John McCain (R-AZ) was fundraising in Richmond, VA, and joked about how he vets prospective VP candidates:

“We’re going through a process where you get a whole bunch of names, and ya — well, basically, it’s a Google. You just, you know, what you can find out now on the Internet. It’s remarkable, you know.”

So, one the one hand, we have Barack Obama who’s unveiled an exceptional “innovation agenda,” shaped in large part by technology visionary Lawrence Lessig. And on the other we have John McCain, who doesn’t know what Google is.

Now, the easy joke is to make fun of McCain running to be the oldest president in American history, and note that his talk about William Jennings Bryan and Jimmy Carter, and his unfamiliarity with staples like Google, is evidence of a candidate who can’t compete in the 21st century.

But as the campaign continued to unfold, this may prove to be a more substantive area for debate. I’d argue we need a leader who’s in touch with modern trends and technology. Who knows what email, networks, search engines, and net neutrality are. Who doesn’t look at modern life as something foreign, and the Internet as something scary.

This isn’t just about physical age; plenty of older people are culturally and technologically savvy. It’s about a candidate who seems more comfortable in the past, and lacks a vision for the future.