Arthur in the comments to this post poses the question. Wolfwalker takes a stab at it, and as you'd expect brings teh Smart. But Arthur's question is a good one. I actually think that there's not a lot to the answer.
What I don't expect is for the Left to become less tribal. Actually, there's no need for them to try, and it would probably be futile, anyway. Hundreds of thousands of years of evolution have made Mankind a tribal animal, and to flout this is to feed our own frustration.
I also don't believe that specific policy positions are key here. I Want A New Left covers this in detail, but I think that the Left can recover even with some losers in the policy bucket.
So what would it take? Having thought about it, I think only three things are needed. Simple (although we'll come back to this).
The Left lives in a world where it wants to have its cake and eat it too. I can't blame them for that - I mean, we all want this - but the rest of us realize that we can't expect life to work this way. This is the first major area where the MSM makes the left feeble, by allowing them to think that they can get away with a double standard, that they can expect one standard by which their side gets measured, and a different one for their enemies.
The Internet has entirely shattered the MSM's ability to control this dialog, but the Left got soft and lazy back when the media covered for them. Now there's no place to hide, and so it's plain for anyone to see when someone is talking out of both sides of their mouths.
The "Sarah Palin is responsible for the Tucson murders" meme is only the latest in a long line of inconsistency. The Left (rightly) puffs itself up about, say, the McCarthy period, where lots of people were unfairly tarred with collective guilt. It was wrong then, as everyone is taught in school. Well, it's wrong today, too.
I don't bring this up because it's immoral to use this sort of double standard against your opponents, although it is. The problem for the Left is that it's ineffective. Alinsky may have been right in the 1960s when the MSM would cover for your tribe, but quite frankly none of you are smart enough to really pull it off in the Internet Age where information wants to - and will - be free. Let me repeat: nobody on the Left is smart enough to pull this off, as the New York Times has discovered to its dismay.
So give it up, because you'll have to become stronger if you become consistent. Palin (or Reagan) is dumb, Tea Partiers are racists, Bill Clinton is a feminist - these tropes are staples of the Left, and are blather of a quite shockingly low quality. While they may be very fun indeed on the Upper West Side, they're holding the Left back. As you become more consistent, your arguments will get less stupid, and you'll start convincing more people.
Police your own.
It's a cliche that the Left refuses to maintain any sense of decorum on its own side - pas d'enemies a gauche. The problem is that the MSM is no longer able to cover up the indiscretions of their tribe. Look, we're going to find out that Bill Clinton is a philandering SOB. We're going to find out that the Tea Partiers didn't shout racial epithets at the Congressional Black Caucus. We're going to find out that the Tucson murderer didn't listen to Talk Radio or Sarah Palin. We're going to find out that Global Warming doesn't cause record cold winters.
I don't expect you not to be tribal, just to apply the same standards to your own tribe (or close to the same, at least) as you apply to the other tribe. We're going to find out anyway.
This sort of scortched earth tribalism (e.g. the National Organization of Women sticking up for Bill Clinton, of all people) may be good tactics, and effective signalling behavior to others in your tribe. But in the long run, it's what's led you to the current disastrous state for the Left: very few women self identify as feminists, many fewer people self identify as environmentalists, only around 20% of people self identify as Democrats. Each time you tell yourself that while he may be an SOB, he's our SOB, another potential supporter shifts from your column to the undecided middle, or from the middle to the other tribe's column.
I appeal to the Left's intelligence here - how do you really think you look calling Sarah Palin dumb while ignoring Joe Biden? You make yourself look absurd, and weak.
People figure this sort of thing out, because in the Internet Age information wants to - and will - be free. So call out your own. You'll be quite shocked at how much support this will get you.
Stick up for Good Governance.
You need the first two for this to be credible, but (a) the Left is the tribe of Big Government, so it has a vested interest in Good Governance, and (b) the general population wants government to work reasonably well. This is potentially a huge winner in the ballot box, but only if people believe that the Left will push the Government to govern in society's interest.
It's the Left that should be leading the charge against Regulatory Capture (but isn't). It's the Left that should be leading the charge to fix broken entitlement programs (but isn't). It's the Left that should be leading the charge to increase the efficiency of governmental agencies (but isn't).
And that's an enormous problem for the Left, in a very practical sense. Every time they ask society to, say, let the EPA assume massive regulatory control over energy consumption, they run bang up against most people's gut feel that the EPA are a bunch of dolts, or in thrall to the Environmental Movement, or in thrall to both the Environmental Movement and Big Energy. Until the Left polices its own here, they won't have - or deserve - the voter's trust.
That's all. Like I said, simples. Ah, but there's the rub. As Clauswitz said, everything in war is simple, but the simplest thing is difficult. And so too, here. The Left has tried to cheat its way through for a long, long time now. It's used to it, and used to the MSM covering for them. They're used to opposing views being suppressed at the University.
They wouldn't stand for this, if it were being done for the benefit of their enemies. Be consistent. All this makes them intellectually weak. Police your own. The proof of the pudding is in the eating, and the proof of your philosophy of Big Government is in a government that works effectively. Insist on good governance. This would make your tribe much, much stronger.
Quite frankly, nothing here would be considered radical to John Maynard Keynes, John Kenneth Galbraith, or even Daniel Patrick Moynahan. People know this - everyone but you, it seems.
This post is long enough already, so I'll apply what this means to different policy positions in another post.