Political scientists call this model a "Downsian" conception of politics (referring to its formalization in Anthony Downs's 1957 book An Economic Theory of Democracy) but even non-academics tend to accept its basic premises—which is why ideologically moderate candidates are generally viewed by political pundits as having a stronger chance of election in two-party competition than relatively extreme rivals. One important implication of Downian logic is that the median voters, or median legislators, on this ideological spectrum wield decisive political power, because they are strategically positioned to dictate the ultimate policy outcome. Thus we can end up with moderate policy even when moderates represent a minority of the total population of political actors.
When the Democrats were debating the ACA in 2009 and 2010, they had to pay attention to the demands of the moderate bloc because moderates held the pivotal votes in Congress. There could be 200 Democrats who favored a provision—like the public option—and 30 who opposed it, but the 30 could get their way over the wishes of the 200 because they could always threaten to join Republicans in a majority that would vote down any bill they viewed as too liberal. This is a familiar strategic environment for vote-counting party leaders, and jibes with the intuition of many political analysts.
But the House Republican Party does not really work this way. The members of the House Republican Conference who are the most liable to threaten defection—and to deliver on such threats—are the hard-line conservatives in the House Freedom Caucus. It was the Freedom Caucus that torpedoed Round 1 of the ACA repeal in March, on the grounds that the bill did not go far enough in a conservative direction. And it was the support of the Freedom Caucus this afternoon that allowed the second effort at repeal to narrowly squeak through the House, after winning concessions in the interim that pushed the bill further to the ideological right.
One might expect that satisfying the demands of the Freedom Caucus would doom the bill's support among Republicans representing politically marginal districts. But it turned out that while many of those members communicated great discomfort with its provisions, they were not willing to withstand the political blowback from within their own party by supplying the key votes to kill the bill.
The House GOP is thus in an unusual position in which the pivotal policy influence in the caucus lies on the party's right edge, not in its center. Thus the bill picked up greater support as it moved further in a conservative direction over time—a pattern that is directly inconsistent with traditional legislative logic. Even Republicans from competitive districts became more supportive of the AHCA as it shifted to the ideological right; while they were willing to pile on against the previous version once the Freedom Caucus had already vowed to block it, they were substantially less enthusiastic about courting conservative attacks by opposing the bill from the left once their own votes would prove decisive to the outcome.
It should be noted that the Republican Party's frequent rejection of Downsian logic extends to the electoral sphere as well. Rather than view voting for the AHCA as an unacceptable risk given the law's unpopularity among swing voters, many Republicans believed that they would court greater danger by failing to pass anything and thus demobilizing their own party base:
GOP consultant: "House majority is in greater jeopardy if we DONT pass this than if we do... the bottom falls out if we don't get something"— Jake Tapper (@jaketapper) May 4, 2017