Some Republicans argue that the closeness of the race — especially against a candidate with Trump’s vulnerabilities — is evidence of Clinton’s fundamental flaws as a candidate.

“She’s not up more because she’s disliked and distrusted,” said GOP consultant Rick Wilson, who is working with independent presidential candidate Evan McMullin. “With any other candidate than [Clinton], Trump would be down by 25 percent.”

Other experts highlight additional factors. One challenge for any candidate is the degree to which viewing habits have changed, with audiences fragmenting and many young voters, in particular, moving away from traditional TV viewing.

“There is no question that the return on investment for paid advertising continues to degrade from election to election, as it does for commercial TV advertising generally,” said Chris Lehane, a Democratic strategist who served in former President Bill Clinton’s White House and on then-Vice President Al Gore’s 2000 campaign.