Clinton allies, for example, see an opportunity to win the battleground state of North Carolina because of Mr. Trump’s sharply unfavorable ratings with women.  The Trump campaign believes the number of white Democratic voters who find Mrs. Clinton untrustworthy could help tip Ohio and Pennsylvania into its column.  “This election is not going to be about issues; it’s going to be a race about character and temperament between two of the most unpopular political candidates in history,” said Steve Schmidt, who managed John McCain’s 2008 presidential race.  Even though Mrs. Clinton and Mr. Trump are among the best-known people in America, their unpopularity with the general electorate, as shown in polls, does not mean they can’t improve their images. Both President Obama in 2012 and George W. Bush in 2004 entered their re-election races with higher unfavorable ratings than their opponents but ultimately won.  The candidates’ selections of running mates, the national conventions and especially the three presidential debates in September and October will offer broad new canvases on which to create impressions.  Mr. Trump’s campaign manager, Corey Lewandowski, said his approval ratings will improve as a wider universe of voters gets to know him.  “Look at the nationwide polls for Mr. Trump right now: His favorable/unfavorables have continuously improved,” he said. “The more people that see and meet Donald Trump, the better he’s going to do.”  Mr. Trump was viewed unfavorably by 57 percent of voters nationally in a CNN/ORC poll released this month, down from 67 percent in March, but he is still registering historic levels of unpopularity.  Mrs. Clinton was viewed unfavorably by 49 percent in the recent poll, an improvement from 56 percent in March.  “Democrats dislike Trump and Republicans dislike Hillary, but Hillary is better liked among the Democrats than Trump is among the Republicans,” said Mr. Goeas, who worked for an outside Republican group opposed to Mr. Trump but said he would now support him as the presumptive nominee.  “The first thing he needs to do is fix his problem with Republicans,” Mr. Goeas said. “The judgment on him has been passed on his personality, not his position on issues.”  Mr. Trump’s efforts to unify his party got off to a perilous start last week, with a former rival in the primary, Jeb Bush, saying he would not endorse him and the House speaker, Paul D. Ryan, saying he was “just not ready” to back him.  The support of Republican leaders matters less to Mr. Trump than that of regular voters in a year in which he has channeled grass-roots fury at the establishment.  But his effort to broaden support will soon face a barrage of negative television and web ads from a super PAC backing Mrs. Clinton, which promises much tougher criticism than Mr. Trump’s Republican rivals delivered.  “It would be a huge mistake to think we’ve had a real-world litigation of any of these criticisms of Trump,” said Geoff Garin, a pollster for the group, Priorities USA Action.  The group plans $125 million worth of ads in the runup to Election Day. They will run in seven battleground states, emphasizing that Mr. Trump has “made his billions on the backs of others” and has a “temperament that’s ill suited to keep America safe,” said a spokesman, Justin Barasky.  “In some ways Hillary’s negatives are already baked in the cake, and that is simply not the case with Trump,” Mr. Garin said. “Virtually all of Trump’s negatives are the product of his bizarre conduct, as opposed to any specific set of facts people have at their command about his record or what the course of a Trump presidency is likely to be.”  Mr. Trump has been foreshadowing attacks on Mrs. Clinton’s temperament. He is highlighting what he claims is her lack of truthfulness about the attacks on Americans in Benghazi, Libya, an issue reflected in the high number of voters who tell pollsters she is untrustworthy.  “When they called her on Benghazi, she was sleeping, folks,” Mr. Trump told voters in Indiana last week.  The Trump campaign is well aware that it needs to work to close a steep deficit with women, and that for many independent women, national security is the top issue. “Women are looking for security in our country, and they know I’m going to do the best job,” Mr. Trump said in Indiana.  The Clinton campaign foreshadowed last week that it would direct withering fire at Mr. Trump’s personality. It released a pair of web ads, one using the words of his former Republican rivals against him (sample: “utterly amoral”), the other including a comment that he would be willing to use nuclear weapons against the Islamic State, even in Europe.  “You have to have people imagine Trump behind the desk in the Oval Office and have them be scared,” said Dan Pfeiffer, a former senior adviser to Mr. Obama.  The Electoral College map will dictate the terms of engagement. Mr. Trump is counting on his appeal to blue-collar workers in the Rust Belt. After he became virtually assured of the nomination last week, he spoke of the “carnage” that trade deals wrecked on American manufacturing. His campaign believes the issue will put the industrial states of Michigan, Pennsylvania and Wisconsin into play, where no Republican nominee has won since the 1980s.  “He has excited a base of people that have not been excited before,” Mr. Lewandowski said.  But Mr. Trump’s deep unpopularity with Hispanic voters, after labeling Mexican migrants criminals and calling for a “deportation force,” complicates his prospects in the swing states of Colorado, Nevada, New Mexico and, especially, Florida.  A CBS News poll in April measured his unpopularity among Hispanics at 82 percent.  “Many Hispanic voters see this as a watershed election, where Donald Trump has put the good name of their community on the ballot,” Mr. Garin said.
Each candidate hopes to exploit the weaknesses of an unpopular opponent in a race that promises to center on personality and temperament more than issues.