The last five years in Western politics has seen a repeated failure of “cordons sanitaires” — the barriers that political establishments have tried to throw up against both radical ideas and xenophobic sentiments. The rise of populism and the return of socialism have breached these cordons, and racism and Judeophobia have come through the breach with them — to the point where it’s entirely plausible that Britain will soon find itself with a prime minister, the Labour leader Jeremy Corbyn, who has an anti-Semitism problem, even as the United States has already elected a birther to the presidency.

This week we’re watching two interesting attempts to re-establish or shore up those old barriers. On the right, there’s the congressional Republican effort to isolate Steve King, the Iowa congressman whose racist comments and flirtations with white nationalism have become, at long last, a matter of severe embarrassment to his colleagues. On the left, there’s the mass exodus of corporate and political sponsors from this weekend’s Women’s March, which has fallen into controversy because of some of its leaders’ ties to Louis Farrakhan, and reports that anti-Semitic canards were aired at its organizing meetings.

The two efforts are similar but not parallel. The push against King is an attempt to redraw a line effaced by Donald Trump’s race-baiting, and since as you may have noticed Trump is still the president, it matters only as a possible marker for a post-Trump Republican future, not a defining statement for the G.O.P. today. The exodus from the Women’s March, on the other hand, is an attempt to get out ahead of a problem before it becomes worse — before anti-Semitism migrates from the left-wing fringe to the center, before the party starts getting its own versions of Jeremy Corbyn in positions of real influence.

This is not to say that anti-Semitism or other paranoid worldviews are a new problem on the left, or that the Democratic Party has always handled them effectively. The permanent prominence of Al Sharpton and the eternal return of Michael Moore testify to certain unsuccessful reckonings, and the grass-roots left can be as amenable to conspiracy theories as the grass-roots right. But the fact that Trump is in the White House while the Democratic National Committee bails on the Women’s March illustrates a fundamental difference; if the Democrats struggle with the tiger, the Republicans have let it leap the cage.