Harry Enten looks at statistics that rank members of Congress on a scale from -1 for most liberal to 1 for most conservative and finds Senate and House Democrats have been fairly stable at -0.4 since 1992.
“There has, however, been an increase in partisanship in the House, and it truly is ‘asymmetrical’. The Republican House caucus has been becoming more conservative every year since 1977, whether or not House Republicans are winning or losing elections. Republicans have climbed from 0.4 on the DW nominate scales after the 1992 elections to near 0.7 in the last congress. That type of charge towards polarization is historically unusual over data that stretches back 130 years.”
Likewise, Senate Republicans “have slowly and become more conservative in their roll call votes by moving from about 0.3 to 0.5 on the scale.”