Jan. Mar. May July Sept. Nov. +40 +20 Bush ’00 Dole McCain Bush ’04 0 Romney Net favorability Republican nominees Bush ’92 -20 Donald J. Trump -40 Jan. Mar. May July Sept. Nov. +40 +20 Bush ’00 Dole McCain Bush ’04 0 Romney Bush ’92 Net favorability Republican nominees -20 Donald J. Trump -40 Jan. Mar. May July Sept. Nov. +40 +20 Bush ’00 Dole McCain Bush ’04 0 Romney Net favorability Republican nominees Bush ’92 -20 Donald J. Trump -40 Jan. Mar. May July Sept. Nov. +40 Obama ’08 +20 Net favorability Bush ’00 Clinton ’92 0 Hillary Clinton -20 Donald J. Trump -40

Jan. Mar. May July Sept. Nov. +40 Democratic nominees Obama ’08 +20 Gore Clinton ’96 Obama ’12 Kerry Clinton ’92 0 Hillary Clinton -20 Jan. Mar. May July Sept. Nov. +40 Democratic nominees Obama ’08 +20 Gore Clinton ’96 Obama ’12 Kerry Clinton ’92 0 Hillary Clinton -20 Jan. Mar. May July Sept. Nov. +40 Democratic nominees Obama ’08 +20 Gore Clinton ’96 Obama ’12 Kerry Clinton ’92 0 Hillary Clinton -20

The election is, in fact, a popularity contest. “The candidate with the highest favorable ratings generally wins,” said Frank Newport, editor in chief of Gallup.

Political scientists cite several reasons for Mr. Trump’s and Mrs. Clinton’s record-low popularity, including Mrs. Clinton’s continued opposition from Senator Bernie Sanders of Vermont and the divisiveness Mr. Trump has created within the Republican party.

In addition, American views of the parties and the candidates are more strongly divided along party lines than in the past. Political scientists say this has resulted in a phenomenon where people vote against the opposing party rather than for their own candidate.

“We are in a position where both parties have extremely negative views of the opposite party and that pulls down the candidates’ favorables,” said Charles Franklin, director of the Marquette Law School poll. “The good news for both of them is they have to run against each other. So, in that sense, neither is substantially disadvantaged.”

Within the Parties, Cautious Optimism

+60 Hillary Clinton Net favorability by candidate’s party +30 Becomes presumptive nominee Donald J. Trump 0 Aug. ’15 Sept. Oct. Nov. Dec. Jan. ’16 Feb. March April May +60 Hillary Clinton Net favorability by candidate’s party +30 Donald J. Trump Sept. Nov. March May Jan. ’16

Another bit of good news for both candidates is that the two candidates are decidedly more popular among their parties’ faithfuls.

“The image of the candidate among his or her own party is a critical metric because it can be an indicator of turnout,” Mr. Newport said.

Despite their net positives among partisans, both candidates are faring worse than their recent predecessors, though experts predict that could change.

“Mr. Trump has been an unusually divisive nominee, but I think the Gallup data since early April is showing pretty conclusively that the party is coming together behind him,” Mr. Franklin said.

Likewise, Mrs. Clinton needs Mr. Sanders to drop out in order for the Democratic party to unite behind her.

Jan. Mar. May July Sept. Nov. +40 Bill Clinton ’92 convention +20 John McCain convention Al Gore convention 0 Net favorability among all adults -20 Jan. Mar. May July Sept. Nov. +40 Bill Clinton ’92 convention +20 John McCain convention Al Gore convention 0 Net favorability among all adults -20

What to Expect From the Conventions

With 160 days left until Election Day, the national party conventions this summer could be the candidates’ best chance to increase their popularity. Mr. Sanders’s insistence on remaining in the race through the convention could temper Mrs. Clinton’s potential convention bounce, the lift she would get once the party’s convention ends.

Bill Clinton was the master of the convention bounce in 1992; Al Gore got a lift in 2000.

The 2008 Republican convention gave John McCain a notable increase, but it did not last. The Democratic convention that year unified Democrats behind Barack Obama after a bruising primary battle with Mrs. Clinton.