After a raucous Republican convention nominated the very conservative Barry Goldwater in 1964, President Lyndon Johnson’s campaign ran an advertisement quoting William Scranton, Pennsylvania’s moderate governor, describing “Goldwaterism” as a “crazy-quilt collection of absurd and dangerous positions.”

Welcome to what will certainly be one of the central themes of the Democratic National Convention. Donald Trump’s nomination at a dark and angry convention in Cleveland and his acceptance speech embracing a racially tinged authoritarian nationalism open up a wealth of opportunities for Hillary Clinton’s campaign.

This is the week in which Clinton could nail down the support of the nation’s Latino and African American voters while sowing deep doubts about Trump among what is likely to be the election’s key target group: college-educated white voters.

She reinforced her appeal to them by picking Tim Kaine as her running mate. He’s thoughtful, experienced and respected, broadly progressive yet with a moderate, conciliatory demeanor.

Sen. Tim Kaine (D-Va.) is Democratic presidential candidate Hillary Clinton's vice presidential candidate. Here's what you need to know about him. (Sarah Parnass,Osman Malik/The Washington Post)

And before the controversy over anti-Bernie Sanders emails from Democratic National Committee staffers could dominate the early part of the convention, Clinton’s campaign moved quickly to obtain party chair Debbie Wasserman Schultz’s resignation.

Clinton had to muffle the story because she has real work to do this week. The Democrats’ conclave will be far more positive and upbeat than the GOP’s gloomy attack-fest because one of its central objectives will be to boost Clinton’s favorable ratings. They took a tumble after FBI Director James B. Comey’s verbal excoriation of her use of a private email server.

Democrats will be battling what they see as a false equivalence in the media that casts both major-party candidates in the same light because of surveys giving each of them historically high negative scores. Clinton’s campaign wants Democrats (who will form a large part of the television audience) to come away with new enthusiasm for their candidate, and swing voters to see Clinton as far more ready than Trump, by experience and temperament, to be president.

Accentuating the positive will also be important because Trump has bet his candidacy on his ability to persuade a sufficient share of the electorate that the nation really is in the midst of a catastrophic crisis.

Here is where the minority of Americans who pay close attention to both conventions will suffer from an acute case of whiplash: Democrats will not only be arguing that Clinton offers a better future; they will be vigorously defending President Obama’s legacy.

Republicans may come to regret their decision to harness Clinton and Obama together as twin authors of a national apocalypse. At a time when the president’s approval ratings have been healthy, the GOP helped lock in Obama’s strongest supporters behind the woman who had once been his political adversary.

The ferocity of Trump’s attacks on Obama paradoxically make it easier for Clinton to advance the dual-track case she needs to make: that she will build on rather than demolish the president’s achievements while also tending to long-standing problems that predated the Obama years. The GOP’s picture of Obama is a wildly distorted parody, and parodies are more vulnerable to the facts than are honest descriptions of reality.

This convention will also be an opportunity to offer a gentle reminder that the last time someone named Clinton was president, the nation enjoyed a run of peace and prosperity. During the GOP gathering, Sen. Jeff Sessions (R-Ala.) declared that incomes had not recovered since their high in 1999. Trump made the same point using the year 2000. Neither mentioned who was president back then.

But the Philadelphia Democrats have a moral obligation as well: They cannot concede the white working class to Donald Trump.

Sanders, who speaks Monday, will play a vital role in seeing that they don’t, and shrewd vote counters know that surrendering this constituency would endanger Clinton in states such as Pennsylvania, Ohio and Michigan. But more than calculation is involved. Democrats have a responsibility to unite a fractured nation. The pain faced by those who work for wages transcends the lines of race and ethnicity.

At stake are the party’s oldest commitments. When the Democrats last met in Philadelphia, in 1948, President Harry S Truman insisted it was their party that had served as “the haven of the ordinary people of this land and not of the favored classes or the powerful few.” It was here 12 years earlier that Franklin Roosevelt declared: “Liberty requires opportunity to make a living — a living decent according to the standard of the time, a living which gives man not only enough to live by, but something to live for.”

Clinton has to cut through the static surrounding her to persuade those whom Trump is wooing with the politics of fear that she and her party still offer a credible politics of hope.

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