The phrase "vulnerable incumbent" refers to a person currently in a position of power or authority who is at risk of losing their position due to various reasons, such as strong competition or unfavorable circumstances. Full definition
The program is designed to help raise money and assist vulnerable incumbents seeking re-election. (ballotpedia.org)
Comstock — seen as one of the most vulnerable incumbents in November — declined to attend, but Virginia Sens. Tim Kaine and Mark Warner, both Democrats, and two Democrats vying to replace her, Alison Freeman and Dan Helmer, did. (local10.com)
The Democratic Congressional Campaign Committee on Monday is naming 19 members to the Frontline Program for its most vulnerable incumbents in 2018. (rollcall.com)