From Gallup:
After hitting a new high last month, mentions of immigration as the most important problem facing the U.S. increased further to 27% in July. Since Gallup began regularly recording mentions of the issue in 1993, immigration has been cited by an average of 6% of Americans, though it has been higher in recent years. There have been occasional, typically short-lived, spikes when major immigration events were occurring.
The July 1-12 poll was conducted as the U.S. government continues to struggle to handle the large number of Central American immigrants attempting to enter the U.S. via the U.S.-Mexico border. The issue was brought into sharper focus in early July when Democratic congressional leaders and Republican Vice President Mike Pence made separate trips to facilities that are holding the migrants as they await asylum hearings.