Based upon the Preface, Introduction, and chapters 1-4 of Ian Haney López's Dog Whistle Politics: How Coded Racial Appeals Have Reinvented...

When Richard Nixon realized in 1968 that he was losing Republican support to George Wallace, who was running as an American Independent Party candidate, he "opted to tack right on race," as Lopez writes. 

Nixon made a "backroom deal" with Senator Strom Thurmond from South Carolina, a strong segregationist who had supported Goldwater. He secretly promised the senator that if he were elected, he would restrict school desegregation. Once he was elected, as promised, President Nixon...

When Richard Nixon realized in 1968 that he was losing Republican support to George Wallace, who was running as an American Independent Party candidate, he "opted to tack right on race," as Lopez writes. 

Nixon made a "backroom deal" with Senator Strom Thurmond from South Carolina, a strong segregationist who had supported Goldwater. He secretly promised the senator that if he were elected, he would restrict school desegregation. Once he was elected, as promised, President Nixon took a stand against "forced busing," in which children were bused sometimes up to an hour's trip across a city to another school in order to establish integration.
Lopez further claims that Nixon took a racial stand when he "hammer[ed] away at the issue of law and order" ("dog whistle") as Southerners protested against racial activists as "lawbreakers" since they violated Jim Crow statutes.

Dismissing these protesters as criminals shifted the issue from a defense of white supremacy to a more neutral-seeming concern with "order" while simultaneously stripping the activists of moral stature.

With the issue of law and order at the forefront, there was, then, justification for making arrests of the civil rights activists for "trespassing and delinquency." So, by using the "dog whistle" phrases, "forced busing," "law and order," and "protection from unrest" Nixon was able to subliminally appeal to the anti-black voters. 

When George Romney, Nixon's secretary of housing and urban development planned to integrate the suburbs by cutting federal funds to any community that refused to integrate, his plan was arrested by President Nixon, who declared that forced integration was not in the "national interest." Lopez comments, "That dog whistle blasted like the shriek of an onrushing train."