Democrats and Republicans in the US Congress are as ideologically consistent and distinct as they have been at any point in the past three decades. Since 1973, the Senate has become 29 percent more polarized and House polarization has increased 47 percent. This investigation of party polarization finds that member adaptation accounts for one-third of the total party polarization in both the House and Senate. Member replacement accounts for the other two-thirds, the lion's share of which has been the replacement of moderate southern Democrats by conservative Republicans. Republicans in both chambers are polarizing more quickly than Democrats. If the Democratic senators have taken one step toward their ideological home, House Democrats have taken two steps, Senate Republicans three steps and House Republicans four steps.
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