The answer to this question can be found in Chapter Five of Dog Whistle Politics, which actually deals more with the racism in the Clinton and George W. Bush presidencies. Lopez presents the elder George Bush as the product of moderate Northeastern conservative Republicans, who had generally eschewed race-baiting as a political tactic. But Bush was also faced with the possibility of losing the 1988 election to Michael Dukakis, and his chief campaign advisor, Lee Atwater persuaded him that he had "plotted a racial route to victory," and the genteel Bush accepted his advice. 

Lopez focuses on the Bush campaign's notorious Willie Horton advertisement as the means by which race became a pivotal feature of the election. Horton, imprisoned for murder, broke into a house, stabbed a man, and raped his partner while on furlough. Michael Dukakis had, while Massachusetts governor, vetoed a measure that would have denied furlough to convicted murderers, and Atwater played on this fact to emphasize that the governor was "soft on crime." But more important to Lopez's argument, Atwater and Bush's other advisors sought to play on the fact that Horton was black and his victims white. The author quotes an unnamed Bush advisor as gloating that Horton case, and its notoriety in the media, represented a "wonderful mix of [white] liberalism and a big black rapist." He goes on to quote political observers who described Horton as Bush's most important ally in the election, which he won, and claims that the Horton ad represented a crucial turning point.

By citing this case, Lopez is attempting to demonstrate what he views as the centrality of "dog whistle" politics to modern conservatism. The Bush campaign could not, of course, have made explicitly racist statements. These would have offended most Americans, even those in the conservative base. But Lopez argues that by framing an issue in racist terms (still quite explicit), the Bush campaign raised an issue that evoked white racialized fears. Like a dog that can hear a whistle inaudible to people, many white people "heard" the message in the Horton ad in ways that mobilized them against Dukakis for reasons that Lopez views as racist.

According to Dog Whistle Politics: How Coded Racial Appeals Have Reinvented Racism and Wrecked the Middle Class, George H.W. Bush originally rejected the idea of using race in his 1988 presidential campaign.  However his campaign manager, Lee Atwater, convinced him race could be the key to his victory.

Willie Horton was the perfect criminal to help Bush’s poll numbers.  Horton escaped while on furlough, raped a woman and stabbed and bound her fiancee.  Michael Dukakis, the Democratic presidential candidate, vetoed a bill that would make murderers ineligible for furloughs.  Bush used this opportunity to blame Dukakis for this horrible crime. 

Atwater began using the Horton case in political ads.  He never blatantly mentioned race, but knew how the public would react to a black male raping a white female in the presence of her fiancee.  Bush, who was once lagging in the polls quickly gained more supporters.   In the month following the ads, Bush gained 12 percent of the Dukakis supporters.