And why don't WE have someone with this kind of power to enforce discipline?

I'll offer a few thoughts on that.

First, as has often been noted, the Democratic party is a bigger tent than the Republican party.  This has its advantages and its drawbacks.  One of the latter is that efforts to "enforce" particular policy commitments (and which would those be?  Defending Social Security and Medicare, up until now perhaps, might come closest) run what is arguably a greater risk of driving people out of the party.  See my second point below, about Democrats being less authoritarian than Republicans.  Democrats are more likely to bolt or not join the party, and react defiantly to authorities seen as trying to enforce a party line they don't agree with on a particular item important to them than are Republicans.  You'd have thought that with all the anti-gay marriage and same-sex partner, and "abortion is murder and should be criminalized", energy on the Republican side that there would be far more open talk from the most committed advocates on these issues of leaving the Republican party given what, to their way of thinking, have been deeply disappointing results.  But not so.  They stay in the tent and they have moved policy on some hot-button social issues, abortion policy notably, farther in the direction they want, over decades.

Second, Republicans and conservatives are more authoritarian in their outlook and behavior than are Democrats and liberals.  (I highly recommend Authoritarianism and Polarization in American Politics, 2009, by Marc Hetherington and Jonathan Weiler for backup on that claim.)  Put another way, Republicans are generally just more willing to fall in behind their leaders and accept direction and authority than are Democrats.  Bill Clinton made a similar point when he said (paraphrasing, but close)  "Democrats want to fall in love with their President; Republicans fall in line behind their Presidents." 

I suggest that a quick survey of comments here at dag and at any number of other places in the blogosphere would tend to confirm that view.  Democrats are as a generalization not only more diverse in their views but are more skeptical and more likely to see skepticism, questioning authority, as more of a good than a bad thing.  As it is in many contexts.  Just not necessarily when it comes to building a well-oiled, effective political party that is able to act more in concert with one another than its opposition is able to do.

One of the ironies of Hetherington's empirical findings is that the Republican cartoon caricatures of Democrats as somehow this menacing would-be totalitarian, liberty-stripping crowd would be comical if they weren't so effective in mobilizing believing party supporters peddling that nonsense.  The opposite is closer to the truth.  Democrats are much closer to anarchy than totalitarianism, not just in the way they(we) function, but in what we believe and in our greater apparent valuation of, and interest in, dissent (Will Rogers' quip about not belonging to any organized political party, being a Democrat as he was).  Democrats are barely able to agree on anything in any way that matters, that results in widespread citizen actions that move politicians. 

Where Democrats do come together it is almost always as a defensive reaction, in opposition to some proposed horrible thing the Republicans are trying to do, such as cut Social Security or Medicare.  Democrats rarely get energized and come together and work together closely in support of positive ideas.  There were groups active on health care but no group anything like AARP, say, in numbers or impact.  Where is the massive grassroots group beating down the doors demanding that Congress pass a meaningful, effective carbon emissions bill?  It's only the future of the planet that's at stake, after all.  There are groups doing that, don't get me wrong.  It's just that they are relatively tiny and not in any way seen by politicians as capable of offering either decisive political support or opposition on that issue.     

It's fascinating to me that the political party that philosophically attaches far greater value to social solidarity for the society at large is so lacking in it as a political party.  Unless and until that changes, I think the prospects for getting far on a progressive policy agenda are not terribly promising. 

Genghis wrote awhile back that unless and until Democrats decided on a policy of primarying conservative to moderate incumbents or challengers with progressives the party would continue to fall short of having enough progressive votes in Congress to pass progressive legislation.  I think he was quite correct about that.  Citing the Republican party's history on this and assuming a parallel, he said that Democrats would also have to be prepared to lose many an election having nominated the more progressive candidate, but that over time if the party stuck to that strategy it would work in the sense of making the party more consistently progressive.  In my experience Democrats have a hard enough time agreeing on what they're going to do today and tomorrow, let alone committing to a multi-year, maybe multi-decade plan that they are also committed to and able to implement.  

Perhaps it shouldn't be surprising that the Republican party, as the preferred party of big business, would tend to have more strategic, long-range capacity to make and implement plans.  Doing that is more conducive to the culture of the GOP.  In any case, they've done that in many crucial respects and as a result they have built a formidable infrastructure that supports the party's objectives, such that they don't even need to nominate impressive presidential candidates but regularly are able to make lemon candidates look enough like lemonade to win elections they have no business winning based on their ideas and record.

I think it's because nobody knows what "liberal principles" are any more when it comes to hard, cold bread & butter issues.