Installing a new A.T.F. director is one of the few consequential moves Mr. Biden’s administration can still make. The major policy changes Mr. Biden espoused during the 2020 campaign — including his vow to impose universal background checks on gun buyers — have been blocked by Senate Republicans, working in tandem with the National Rifle Association and other pro-gun organizations.
A year ago, during an earlier wave of mass shootings, the White House tapped David Chipman, a pugnacious opponent of the gun lobby, to run the A.T.F. But administration officials, who were focused on pushing through Mr. Biden’s unsuccessful Build Back Better package, left Mr. Chipman on his own to respond to a fierce backlash. By September, they were forced to withdraw his nomination.
Mr. Biden’s aides then took months to select Mr. Dettelbach, 57, whose confirmation they see as a smoother prospect than that of the brash and confrontational Mr. Chipman: He is upbeat, avoids bombast, and has been guided by administration officials and a Senate sponsor, all anxious to prevent another defeat.
Seven former A.T.F. directors have endorsed him, as have dozens of law enforcement officials around the country, along with the International Association of Chiefs of Police, the largest such organization in the country, and the Federal Law Enforcement Officers Association, the largest such organization representing federal agents, including A.T.F. employees.
Yet he still faces only the narrowest of paths to confirmation.
Republican opposition, even after Tuesday’s events, is likely to be unanimous, and a single Democratic defection will sink Mr. Dettelbach’s nomination. His performance at his confirmation hearing on Wednesday — in the shadow of the deadliest mass shooting of the year — is likely to determine his fate.
“I’ve been talking to a number of Democrats who say how favorably impressed they have been with him, how favorably toward him they feel, but they want to watch the hearings just to make sure,” said Senator Sherrod Brown, Democrat of Ohio and a friend of Mr. Dettelbach’s, who has been speaking with colleagues on his behalf.
“I’m pretty certain we’re going to confirm him,” he added.
Democrats felt nearly as sanguine about Mr. Chipman, a former A.T.F. agent who worked until recently for the Giffords group, after securing the backing of the chamber’s perpetual swing voter — Senator Joe Manchin, the conservative Democrat of West Virginia. Then, over the summer, the National Shooting Sports Foundation, a gun manufacturers’ trade group, began quietly deploying its state-level affiliates to peel away Democratic support.