A lot of recent polls show that about 30% of Clinton-supporters and about 20% of Obama-backers will vote McCain if their preferred Democratic candidate doesn't get the nomination. This basically means that both Senators Clinton and Obama would lose in November!
To avoid that problem, the sooner this nomination is wrapped up, the better - preferably by June 3 (the last Democratic primary). If it were to be over sooner because either candidate (read particularly Senator Clinton) is forced out of the race, that candidate's supporters might feel that their candidate did not get a fair shake. [Even though Senator Clinton proclaimed back in 2007 that the race would be over on Super Tuesday - effectively declaring the post-Feb 5 contests irrelevant. This was evident in her campaign's lack of preparation for these later states.]
Post-June 3, the eventual nominee will have 5 months to unite the Democratic Party - much better than the 2 months that will be available post-Convention. That is crucial to the Democratic candidate's chances, in my opinion.
One of the main reasons some Democrats/Dem-leaning Independents consider voting for Senator McCain is because they think he is centrist. A project of the Democratic Party/supporters (527s!) should be to paint Senator McCain as a Bush Conservative. I think this should be very possible, at least on the following issues:
1. Iraq, Iraq, Iraq - Senator McCain wants to stay in Iraq till a nebulous, undefined "victory" is achieved, continuing Bush-43's policies. Two-thirds of Americans want OUT. [See update below.]
2. Bush tax-cuts - Senator McCain was a Real Conservative when he voted against the Bush tax-cuts, citing the lack of cuts in spending. Now, he is for making the (currently-temporary) Bush tax-cuts permanent - never mind the ever-increasing federal deficit - saying that doing so would mean a tax-hike! The Fiscal Conservative is long-dead... [link]
3. Reproductive rights - Senator McCain is pro-life/anti-reproductive rights (a good read). This is a key issue for many of Senator Clinton's supporters. Do they really want to risk a third GOP term - during which, given all 4 liberal judges are about McCain's age or older, the composition of the US Supreme Court can change and overturn Roe? From the NPR article:
"Nancy Keenan, president of NARAL Pro-Choice America, says her group has always considered McCain pro-life as well. And it's not just abortion, she says.
"He voted against family planning, he voted against the freedom of access to clinic entrances — that was about violence against women in clinics," Keenan says, adding, "He voted against funding for teen pregnancy-prevention programs, and making sure that abstinence only was medically accurate. This is very, very extreme.""
[On the flip side, some pro-life Democrats have endorsed Senator Obama, prominently Senator Bob Casey; that should ease the minds of some Independents/conservative Democrats about Senator Obama's position on these issues.]
The above strategy - of definitively linking Senator McCain to Bush-43 and painting him as a Bush Conservative - is similar to the "liberal" accusations that were thrown at Al Gore and John Kerry. And it does potentially use some wedge issues like #3 - even though it appears Senator McCain is the one out of the mainstream on #3 (see the NPR story, which says even some Republican voters favor legalized abortion).
But as I keep saying - it is time the Democrats played offense, and took the fight to "red" states like Nebraska, Virginia and Kansas, in addition to purple states like Iowa, Nevada, New Mexico, Colorado and New Hampshire. Given the substantial fund-raising advantage either Democrat surely will have against Senator McCain, it is just silly to continue the Kerryesque strategy of focusing on just a few battleground states (see the concentration of candidate visits and money spent on FL, OH and PA).
UPDATE 4/18 PM: Just read an article by Michael Hirsh in Newsweek. Hirsh says Obama '08 is beginning to look like Kerry '04, forced to defend his patriotism (a typical right-wing tack that Senator Clinton is also gleefully indulging in) and reduced to talking about "safe" issues like the economy instead of taking on Senator McCain on national security. Hirsh also points out that Senator Obama has the right perspective on Iraq and Afghanistan - if the US wants to catch Bin Laden, 1-2 brigades (10,000 soldiers) need to be sent to Afghanistan, away from Iraq.
Hirsh also says that the Obama position is increasingly becoming the conventional wisdom even with the military top brass. Thus, Senator Obama is, and has been for a long time, right on the issue of Iraq v. Afghanistan; Senator McCain is just wrong. Unlike Hirsh, however, I firmly believe Senator Obama will not make the Kerry mistake of allowing the GOP to set the terms of the debate.