Political commentators have asserted for months that Donald Trump’s dominance of the Republican presidential field is fueled by his anti-immigrant rhetoric. As Thomas Edsall put it:

Donald Trump’s success is no surprise. The public and the press have focused on his defiant rejection of mannerly rhetoric, his putting into words of what others think privately. But the more important truth is that a half-century of Republican policies on race and immigration have made the party the home of an often angry and resentful white constituency — a constituency that is now politically mobilized in the face of demographic upheaval.

This is a very plausible hypothesis, but one with little comprehensive evidence to date. Now, thanks to a collection of survey data from YouGov, we can show how, and how much, voters’ concern about immigration has helped Trump.

The YouGov data includes 14 surveys that were conducted every week or every other week between June 13 and Nov. 9.  In these surveys, the most frequent question about immigration asks respondents to rate the issue’s importance.

Republicans clearly agree that the issue of immigration is important: around 60 percent consistently say it is “very important.” And believing that immigration is important is correlated with more conservative views on immigration. Among Republicans, those who believe immigration is very important are much more opposed to immigration policies like a path to citizenship — according to data in the Nov. 5-9 YouGov poll.

So if Trump’s appeal is partly about his conservative stands on immigration, his support should be particularly strong among Republicans who say that immigration is important.

That’s exactly what these data suggest:

Before Trump announced his candidacy back in June, opinions about immigration had little influence on support for him. But after his first speech as a presidential candidate harshly criticized Mexican immigrants, voters who believed immigration was important flocked to him.

Although his support has increased among other Republicans as well, those who prioritize immigration continue to be his strongest supporters. Indeed, since September Trump’s support has actually declined among other Republicans, but not among those who prioritize immigration. People who think immigration is very important are now 25 points more likely to support Donald Trump than other Republicans (42 percent to 17 percent).

The YouGov/Economist survey also included several other questions about immigration and immigrants, such as (1) How serious of a problem is illegal immigration? (2) Are immigrants more or less likely to commit crimes? (3) Are immigrants good or bad for the economy? (4) And should undocumented immigrants in the United States be deported?

I combined answers to these questions into a single scale ranging from relatively favorable views of immigrants to relatively unfavorable views. The graph below shows a striking correlation between views of immigrants and preferences in the Republican primary.

The red line suggests that the Republicans most opposed to immigration (27 percent of the Republican sample) are about 50 percentage points more likely to support Trump than the small minority of the party who are strongly sympathetic to immigrants. This does not change even after accounting for other factors, such as self-reported ideology on the liberal-conservative scale, religiosity and other demographics.

The black line in the graph shows that Trump’s strong support from Republicans opposed to immigration comes especially at Marco Rubio’s expense. Rubio has been criticized by his Republican rivals for once supporting comprehensive immigration reform.

Of course, there is a chicken-and-egg question here. Are concerns about immigration helping drive Trump support, or did people decide to support Trump for other reasons and only then align their views on immigration with his views? Americans often change their issue positions and the importance they place on issues based on the positions and priorities of their preferred candidates.

But some evidence suggests that opposition to immigration really is driving support for Trump. In these YouGov polls, there are some respondents who are interviewed more than once. I focused on individuals who were interviewed by YouGov both before and after Trump announced his candidacy. People’s views about immigration before Trump even announced his candidacy should not be directly influenced by their views about Trump.

This analysis confirmed the finding shown above: immigration opinions measured “pre-Trump” were still strongly correlated with support for Trump.

These findings are also consistent with a variety of social science research. Attitudes toward minority groups have long been potent predictors of Americans’ political beliefs. Opposition to immigration in particular may have driven whites from the Democratic Party in recent years. And campaign appeals to racial and ethnic anxieties have often succeeded in activating support for politicians.

Moreover, Larry Bartels’ venerable account of presidential primaries contends that as voters acquire more information about candidates, “the public comes to increasingly evaluate candidates on their political merits, in accordance with longstanding political predispositions.” Attitudes toward immigrants should therefore have become more strongly linked to support for Trump as the intense media coverage of his campaign highlighted his immigration positions.

To be sure, this does not mean that immigration is the only reason why Trump has been leading in Republican primary polls. Nor will Trump necessarily ride an anti-immigrant wave to the Republican nomination.

But these results do suggest that Trump’s position on immigration has made him the preferred candidate of the Republican voters most concerned about immigration.

Michael Tesler is Assistant Professor of Political Science at UC Irvine, co-author of Obama’s Race, and author of the forthcoming, Post-Racial or Most-Racial? Race and Politics in the Obama Era.