Hillary Clinton has a large and perhaps growing lead in the nation and in many of the predominantly white battleground states where Donald Trump was thought to have his best shot, according to a wave of new surveys released in the last two days.

Three national surveys — from Fox, NBC/WSJ and Marist/McClatchy — showed Mrs. Clinton ahead by big margins: 10, 9 and 15 percentage points. It’s the worst polling stretch for a presidential candidate at this stage since John McCain in mid-October of 2008.

It’s a little hard to make sense of it all, in part because the timing is a little unusual. This ought to be the heart of Mrs. Clinton’s post-convention bounce, when polling analysts generally preach caution. On the other hand, Mr. Trump had a tough week of his own making — drawing condemnation from Republican leaders and even causing a few high-profile defections from donors and the conservative media.

The prudent approach is to wait, and see whether Mrs. Clinton’s lead endures for another week or two — after convention bounces usually fade. In the interim, we can cautiously say that there is more reason than usual to think that Mrs. Clinton’s newfound lead represents a meaningful shift in the race, one that would make a comeback for Mr. Trump seem daunting if it holds.