No Republican Senate candidate has been as aggressive in using the Supreme Court nomination of Brett M. Kavanaugh as a political weapon as Josh Hawley, the Missouri attorney general who is in an intensely tight race against Senator Claire McCaskill.

A former Supreme Court clerk, Mr. Hawley made his first campaign commercial about control of the court, and he assailed Ms. McCaskill for refusing to say if she would support Judge Kavanaugh. And after the accusation of sexual assault against Judge Kavanaugh last week, Mr. Hawley denounced Democrats for staging an “ambush.”…

“I think the assault allegations neutralize the Kavanaugh issue,” said state Representative Jay Barnes, a Missouri Republican, echoing the private assessments of a wide range of G.O.P. leaders.

What alarms Republicans is that staunch defenses of Judge Kavanaugh, like the one made by Mr. Hawley, could haunt them if Dr. Blasey makes a compelling case before the committee. One Republican senator, Dean Heller of Nevada, has already come under political attack for describing Dr. Blasey’s allegation as “a hiccup” for Judge Kavanaugh.