It's been noted many times that the conservative Republicans driving the shutdown/debt default crisis are insulated from political repercussions for their actions by the fact that they come from extremely conservative districts where they face little to no risk of getting beaten by a Democrat in a general election. The implicit argument is that safe seats equate to not just ideological extremism, but the kind of procedural extremism we're seeing now. Yet as Eric Boehlert observes, there are lots of Democrats who are just as safe as these Republicans—in fact, there are more Democrats with safe seats, and many of those seats are even safer than Republican safe seats. So why don't the extremely conservative Democrats engage in the same kind of gamesmanship the Tea Party Republicans do, threatening to burn the whole place down unless they get their way?
Before we answer the "why" question, here's what we're talking about. Let's look at the Cook Partisan Voter Index, which sorts congressional districts by how much they lean to one party or another using presidential election results. There are 28 districts with a rating of R+20 or more, meaning the district votes 20 percentage points more for Republican presidential candidates than the nation as a whole does, and 69 districts at R+15 or more. On the Democratic side, there are 58 districts at D+20 or more, and 83 districts at D+15 or more. Almost none of the representatives from any of those districts has to worry about getting beaten by the other party, and nearly all of them are ideologically more to the edges of their party.
1 1. This unequal distribution is also why Republicans can get fewer votes than Democrats in House elections, as they did in 2012, and still hold the House. Through a combination of gerrymandering and where people choose to live, Republicans have distributed themselves more efficiently through congressional districts. With the same number of voters, you can have, say, six urban districts that vote 90 percent for Democrats, or nine districts that vote 60 percent for Republicans. All of them are "safe," yet a 50-50 split in the popular vote would then get Republicans nine seats and Democrats six. This, on a broader scale, is what Republicans have managed over the country as a whole.
The Congressional Progressive Caucus lists 72 House members, and if they wanted to unite to make trouble, they certainly could. So why doesn't it happen? There are a few reasons. First of all, because they're liberals, they want government to do things, unlike Tea Partiers, who generally want government to stop doing things. This means that the threat of a government shutdown or a default is something they'd never employ, because they are sincerely distressed when the government stops doing what it does, while many Tea Partiers are unconcerned or even pleased when the government shuts down.
Furthermore, if you look at who those members are, another big difference emerges between them and the Tea Party. Many Tea Partiers are people who hadn't run for office before 2010, or maybe had served briefly in a state legislature where they were bomb-throwers, not legislators. They won their primaries by promising to be the most conservative, Obama-hating member of Congress the folks of their district had ever seen. In contrast, almost none of the safe Democratic members got elected just by saying that they were the most liberal candidate in their race. Most of them worked their way up through the lower political ranks, getting used to cutting deals, making compromises, and solving problems for constituents. They may be very liberal ideologically, but they're also old-school pols in many ways.
That gives them a practicality that their conservative counterparts don't have. For instance, during the debate over the Affordable Care Act, most of those members of the Progressive Caucus preferred a single-payer system, but they didn't say they'd refuse to support reform unless they got that. They also fought for the inclusion of a public option, but when that got dropped too (just to appease a couple of conservative Democrats in the Senate), they didn't withhold their support for the bill over it. They could have killed Obamacare right then, but they decided to take half a loaf.
That's not only because they are personally disinclined to that kind of all-or-nothing politics, but also because the liberal voters they represent are fundamentally different than the conservative voters the Tea Partiers represent. Multiple polls (see here and here) have found that not only are liberals more likely than conservatives to think the politicians who represent them should compromise rather than stick to their beliefs even if bad results ensue, strong conservatives lean more toward the "stick to your guns" side than moderate conservatives, while strong liberals lean more toward the "compromise" side than modeate liberals.
So if you're a liberal Democrat representing a liberal district, neither you nor your constituents are interested in the kind of high-stakes confrontation we're seeing now. Unfortunately, that also means that the power of liberal Democrats is nothing like the power of conservative Republicans, even though the former are greater in number. If the liberals ever said to Nancy Pelosi, "If we don't get what we want, we're setting fire to this place," she'd know they don't mean it. When conservatives say that to John Boehner, he knows they do.