Robert F. Kennedy Jr. is running for the White House as an independent and abandoning his primary challenge to President Biden. The scion of a Democratic dynasty, he’s aiming to win over voters who hunger for a third option on the ballot.

A Republican, Daron Shaw, who conducts Fox surveys with a Democratic partner, Chris Anderson, said that “third party or independent candidates tend to draw disproportionately from those who are more independent and less engaged, and that checks out” in their recent poll.

The Fox poll found Mr. Kennedy drew 13 percent of Democrats and 11 percent of Republicans. His 16 percent showing was greater than the 8.3 percent the last major third-party candidate, H. Ross Perot, won in 1996, leaving Mr. Biden and Mr. Trump tied at 41 percent.

In his campaign announcement last week at Philadelphia, Mr. Kennedy spoke before a banner reading, “Declare Your Independence.” The candidate leaned into the theme, urging a partisan-free focus on issues ranging from poverty and pollution to addiction and housing costs.

Not so long ago, Americans agreed that these problems needed to be confronted to — as Mr. Kennedy’s uncle, President Kennedy, said — achieve “a rising tide” and “lift all boats.” Today, each party seeks to disqualify the other’s solutions, preaching to a choir where all but the most devout are growing restless in the pews.

“In a two-sided conflict,” Mr. Kennedy told his supporters, “each side depends on the other to define themselves as good guys, in contrast to the other side, of course, the bad guys. … And that’s why we’ve seen both parties sacrificing their core values and the foundational canons of democracy.”

Of course, we have heard this all before. American history is littered with footnotes about third-party candidates who came out strong only to fade. What’s different in this cycle is that the discontent with the political establishment has reached a high watermark.

A Gallup survey released October 4 reflects that 63 percent of American adults agree that the major parties do “such a poor job” that “a third major party is needed.” That is the highest tally since 2003, when Gallup started asking the question.

On October 3, the Monmouth Poll found about two thirds of party members uninspired about their presumptive nominees. Just 37 percent of registered voters, it found, are enthusiastic about President Trump; 32 percent said the same for Mr. Biden.

A similar malaise led voters to give President Trump a try in 2016. Like President Obama — a different kind of outsider — Mr. Trump defeated Secretary Clinton, a Beltway fixture. Mr. Trump was the first president who hadn’t served in government capacity or in the military; Mr. Kennedy would be the second.

In 2020, the establishment struck back with Mr. Biden, but a third of eligible voters still didn’t bother to cast ballots. That both Mr. Trump and Mr. Biden are worried about Mr. Kennedy’s appeal is a sign that he threatens to grow the electoral pie and take bites out of their pieces as well.

“Mr. Kennedy is attracting voters from across the political spectrum,” Mr. Kennedy’s campaign told me. “People who have previously voted for Trump or Biden are now moving toward Kennedy. That’s why Kennedy can win.”

At Philadelphia, it’s important to note that Mr. Kennedy did something that Mr. Trump doesn’t. He spoke about why he left the Democrats. He didn’t make “his decision lightly” and it was “painful” to abandon his family’s party. Explaining this conversion, as President Reagan often did, encourages others to do the same.

Expect the major parties to label Mr. Kennedy a spoiler, but support is earned not owned. As the Green Party candidate, Ralph Nader, said in 2000 when asked if he were “worried about taking votes” from the Democratic nominee, Vice President Gore, “No. I’m worried about Gore taking votes away from me.”

Messrs. Biden and Trump both have Amen Corners, but Mr. Kennedy is seizing his moment to offer a third option to the congregation. Expect him to have success preaching to the record number of Americans who are more tired than ever of hearing the same old hymns.