It means that one political party is getting more partisan (or polarized) than the other -- or that one side is moving closer to its ideological pole than the other.
It's what has happened in Congress over the last five decades, according to a report from the Pew Research Center examining roll call votes of lawmakers beginning in the early 1970s.
Using a system that ranks members of Congress from -1 (most liberal) to 1 (most conservative), Pew found that the average Senate Democrat (-.06) and House Democrat (-.07) have grown only marginally more liberal between the 1970s and today.
