Birtherism quickly became a major campaign issue last week when Donald Trump refused to say that President Obama was born in the United States. Instead, he told The Washington Post’s Robert Costa, “I’ll answer that question at the right time. I just don’t want to answer it yet.”

Apparently the right time was Friday morning. After five years of raising doubts about the president’s birthplace, the Republican presidential nominee finally admitted that “President Barack Obama was born in the United States, period.”

The campaign of Democratic rival Hillary Clinton, however, had already seized upon the opportunity to remind voters that Trump had long been a leader of the “racist birther movement.” Or as Clinton tweeted Thursday night:

She continued her attack after Trump’s concession, tweeting:

Polling data supports this claim that birtherism launched Trump’s political career within the Republican Party. Indeed, consider these graphs from a USA/Today Gallup Poll conducted at the height of birtherism, just days before Obama released his long-form birth certificate on April 27, 2011:

The left-hand side shows the relationship between birther beliefs and Republicans’ evaluations of both Trump and then-House Speaker John A. Boehner (R-Ohio). As of April 2011, Republicans who said Obama was definitely not born in the United States rated Trump nearly 40 percentage points more favorably than Republicans who completely rejected birtherism. By contrast, birtherism was not closely linked to Republicans’ support for Boehner.

Boehner was actually more popular among Republicans in April 2011 than was Trump. Nevertheless, the strongest GOP birthers still rated Trump more than 20 points more favorably than the highest ranking Republican at that time, Boehner.

Birtherism also was significantly correlated with Republicans’ perceptions of how Trump would do as president. The right-hand graph shows that nearly half of the strongest birthers thought that he would make a good or great president. Only a quarter of Republicans who thought Obama was born in the United States believed that.

Meanwhile, birther beliefs were not correlated with Republicans’ perceptions of Romney’s potential performance as president. Overall, Republicans thought Romney would make a better president than Trump.  But the strongest birthers still had slightly more faith in a Trump presidency than a Romney administration.

The numbers at the bottom of the two graphs are just as important as the trends. Those sample sizes for each response category show that a majority of Republicans in April 2011 were unwilling to say that the president was born in the United States. This widespread birtherism within the Republican Party provided Trump a substantial base of support for a potential presidential bid.

This base grew even larger over the past five years. A YouGov Poll from earlier this year showed that 53 percent of Republicans thought Obama was not born in the United States, compared with 21 percent who said he was. Most Republicans have also consistently espoused the closely related belief that Obama is a Muslim in 2016 surveys. Political science research shows that those intertwined beliefs that Obama is foreign-born and Muslim are caused in large part by ethnocentric suspicions of minority groups in general, and by anti-black and anti-Muslim attitudes in particular.

So it’s not surprising that beliefs about Obama’s religion were one of the strongest predictors of support for Trump in the 2016 Republican primaries.

In the American National Election Study, Republicans who thought Obama was a Muslim were 40 percentage points more likely to support Trump than were those who correctly believed that Obama was a Christian. Even in a field of more than 10 candidates, about half of all Republicans who thought the president was Muslim backed Trump before the primaries began.

This isn’t merely a case of Trump supporters changing their beliefs about Obama’s birthplace and/or religion to match Trump’s birtherism. In a prior post, I showed that beliefs about Obama’s religion, as measured in 2008, were an equally strong predictor of support for Trump when these exact same respondents were re-interviewed in April-May 2011. Birtherism was a main reason so many Republicans liked Trump in the first place.

Any account of Trump’s rise to the top of the GOP must start with his origins: exploiting widespread backlash against the country’s first black president by peddling birtherism.

Michael Tesler is associate professor of political science at the University of California at Irvine and author of “Post-Racial or Most-Racial? Race and Politics in the Obama Era.”