With the recent speculation that John McCain's campaign may close-up by September, I did something shocking and did a little bit of research as to when declared and FEC filing candidates closed up their campaigns in the past two cycles. And it really does not make a whole lot of sense for a well-funded and polling above an astrick candidate to drop out in September.
In 2003 the first candidate to drop out was Sen. Graham of Florida. He folded his campaign's tent in the second week of October of 2003. The rest of the field stayed in until after Iowa when Gephardt folded. The vanity candidates continued, as Kerry ran with his momentum until Dean folded and converted Dean for America into the Democracy for America organization after the Wisconsin primary. Graham was a weak candidate with no real fundraising ability, no real natural base of support and minimal ability to garner attention. John McCain has money and the ability to grab attention. So I doubt he'll fold at the same or earlier point in the cycle.
During the 1999 and 2000 primary season, Bill Bradley was the only challenger to Al Gore for the Democrats and his campaign continued well into the primary season. George W. Bush faced significant opposition for the nomination, including his New Hampshire loss. In that primary, three Republicans received delegates, and two more had at least 1,000 votes. We saw a couple of early withdrawals, with Lamar (The Plaid Man) Alexander, and Liddy Dole withdrawing in August and November respectively. Several other candidates also had early withdrawals due to their inability to get past the asterisk stage of polling and fundraising.
So recent Republican history suggests that it it plausible for a candidate with minimal money, minimal traction and minimal pathways forward to drop out in August or September. Democratic history is a bit shakier, as 1992 is the next test case, and my memory is not as good as it should be on that one. But to argue by analogy McCain is a weakened candidate, but he is still stronger than Dole, Alexander, Graham, and the numerous vanity candidates who have dropped out a quarter or more before the first vote. I doubt he drops out early.
But I have digressed a long ways from my original intent of the post --- when do you think candidates will start dropping out on each side?
I'll be shocked if Biden and Dodd are still actively campaigning by Christmas. Kucinich will go on until after Super Tuesday, Gravel should be done soon but who else when drop when?