Sen. Ted Cruz's standing with Republicans has taken a hit following his speech before the Republican National Convention last month during which he refused to endorse Donald Trump as the party's nominee.

According to new poll numbers from Gallup, Cruz's favorability rating among Republican voters fell from 59 percent to 43 percent, a 16-point fall in a month's time. His standing with independents and Democrats, meanwhile, remained relatively unchanged.

Overall, only 29 percent of voters have a favorable view of Cruz, while 58 percent view him unfavorably.

Infamously, Cruz's speech called for Republicans to "vote your conscience" in November, which infuriated Trump supporters and RNC delegates, many of whom rained boos down on him.

This is only the second time Cruz's favorability numbers have been under water with Republicans since he announced his presidential bid last March. After an Indiana primary loss ended his campaign, his unfavorable rating spiked to 45 percent, while only 42 percent viewed him favorably.

Soon after Cruz's speech before the RNC, chatter began to surface about a potential primary challenger to him in the 2018 midterms, when the Texas lawmaker will first face re-election. CNN reported that House Homeland Security Committee chairman Michael McCaul is one name being floated to take him on, while a new poll showed that former Gov. Rick Perry would be in a good position to beat him in a primary contest.