Here, from Daily Kos in one chart, is the history of GOP moderates in the US House of Representatives.
... As Christopher Ingraham notes, the Republican party started moving aggressively away from the center around 1975 and "hasn't looked back."
The Washington Post column by Ingraham has that chart and one more showing that the drift away from center is not two sided. Nor is it a liberal/Democratic phenomenon.
Political scientists have known for years that political polarization is largely a one-sided phenomenon: in recent decades the Republican Party has moved to the right much faster than Democrats have moved to the left. As Thomas Mann of the Brookings Institution has described it, "Republicans have become a radical insurgency—ideologically extreme, contemptuous of the inherited policy regime, scornful of compromise, unpersuaded by conventional understanding of facts, evidence, and science; and dismissive of the legitimacy of their political opposition."
The data backing this claim up are pretty solid. The most widely-used measure of political polarization, a score of ideology based on voting developed by Kenneth Poole and Howard Rosenthal, has shown that the Republicans in the Senate and especially the House have drifted away from the center far more rapidly than Democrats. ...
It's worth pointing out that none of this is happening in a vacuum -- House Republicans are become more extreme because Republican voters are electing more extreme candidates. We see many of these same patterns playing out among the electorate as well, as a massive Pew Research Study demonstrated last year.
There is the gridlock, folks.