The episode represents the latest crack in a long-standing but unofficial policy on Capitol Hill to not get involved in primaries featuring incumbents. And the trend is not just limited to Democrats. An increasing number of House Republicans — including some GOP leaders — have grown more comfortable this cycle picking sides in races where they feel it’s urgent to intervene.

Now lawmakers, aides and strategists in both parties say the pattern will be difficult, if not impossible, to reverse. It’s a shift that reflects the ideological — and anti-establishment — churn taking place in the Donald Trump era, and it’s sparking concern among the old guard about rising intraparty warfare.

“More and more members of Congress are going to look and say ‘rules are rules’ but if in fact there’s a district that’s suffering… we’re going to see a lot more members of Congress supporting challengers,” said Marie Newman, who knocked off longtime Illinois Democratic Rep. Dan Lipinski earlier this year with the backing of several prominent Democrats.