From the GSS:
Democrats and independents who voted for Trump in 2016 were almost as restrictionist as Republicans who voted for him. Non-Democrats who voted for Hillary Clinton were split down the middle on immigration, so Trump’s putatively hardline restrictionist stance doesn’t look like it’s what turned most of them away. And 2016 campaign Clinton was, at least rhetorically, considerably more restrictionist than any of the 20+ Democrat candidates for 2020 are, all of whom want to decriminalize illegal border crossings and provide literally anyone in the entire world free healthcare if they are able to make it to a hospital waiting room.
A recent Harvard-Harris poll finds that 64% of respondents oppose opening up the southern border to additional immigrants while 36% are in favor. Independents oppose it by a 2-to-1 margin. The same poll finds a slight majority favoring president Trump both declaring a state of emergency and building the wall, 52% to 48%.
Though the GOP establishment does everything it can to avoid action on immigration for putative political reasons, the reality is immigration restrictionism is more popular than the Republican party is. Trump’s (alleged) desire to stop the influx is similarly more popular than he is.
GSS variables used: LETIN1A, PARTYID(0-1)(2-4)(5-6), PRES16(1)(2)