JD Vance and Tim Walz both became more popular immediately after being chosen as running mates

Do you have a favorable or an unfavorable opinion of [Tim Walz]? (% of registered voters)
JD Vance, pre-announcement (July 13 - 15, 2024): Economist / YouGov
JD Vance, post-announcement (July 15, 2024): YouGov
Tim Walz, pre-announcement (July 27 - 30, 2024): Economist / YouGov
Tim Walz, post-announcement (August 6, 2024): YouGov

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Note: Responses of don't know and not sure are not shown. Vance's nomination was announced at 3:08 p.m. ET on July 15. Nineteen respondents who started the July 13 - 15 survey after that announcement are not shown. The July 15 and August 6 surveys were launched after those days' running mate announcements. The pre-announcement polls asked for views about Vance and Walz, respectively, along with other public figures. The post-announcement polls asked for views about only Vance or Walz.