The White House and Senate Democrats are struggling to secure support in their own ranks to install David Chipman as head of the Bureau of Alcohol, Tobacco, Firearms and Explosives, a position central to President Biden’s crime-fighting strategy but whose confirmation is getting snarled in gun politics.

No Republicans are expected to vote for Chipman in the evenly split Senate, meaning Senate Majority Leader Charles E. Schumer (D-N.Y.) needs all 50 Democrats to back the nominee for a position that has been enmeshed in controversy since it became subject to Senate confirmation 15 years ago.

A month has passed since Chipman’s nomination deadlocked in the Senate Judiciary Committee, and a handful of Democratic senators from states friendly to gun rights have declined so far to support him, despite his nomination by a president from their party. One of the undecided Democrats raised the prospect Tuesday that Chipman’s nomination could be withdrawn.

The White House emphasized that it is fully supportive of Chipman, who served in ATF’s ranks for more than two decades before joining a gun-control advocacy group led by former congresswoman Gabrielle Giffords (D-Ariz.). Biden selected him in April, three months after taking office — a notably early nomination for an ATF director.

“Not only is he a veteran of the ATF, but he’s somebody who’s been a supporter of smart ­gun-reform measures that could save lives of people across the country,” White House press secretary Jen Psaki said Tuesday. “The president felt quite confident in his qualifications and his ability to lead the agency at a time where it hasn’t been led for many years.”

The last Senate-confirmed ATF director left his post in 2015, and the sensitivity of the position has prevented a permanent leader from being approved since then.

Last week, Attorney General Merrick Garland visited ATF headquarters and urged the swift confirmation of Chipman, saying he was slated to play a leading role in the administration’s fight against gun violence and firearms trafficking.

Chipman so far has sat down with 17 senators as he courts their support, as well as an array of law-enforcement groups, including the International Association of Chiefs of Police, the Federal Law Enforcement Officers Association, the Major County Sheriffs of America, the Major Cities Chiefs Association and the National Sheriffs’ Association, according to a White House official.

But the administration has yet to win over a Senate majority, where the contentious dynamics of gun politics have prevented a bipartisan deal on background checks or even a vote on gun measures passed by the House.

Administration officials are particularly concerned about where Sen. Angus King (I-Maine) will land on the Chipman nomination, according to a person familiar with the process who spoke on the condition of anonymity to discuss a sensitive confirmation battle. King is facing pressure from influential sportsmen’s groups in Maine to oppose Chipman and has tersely declined to elaborate on his thinking when questioned by reporters.

“No, no comment,” King repeated Tuesday. “I’m not announcing anything.”

His fellow Maine senator, Republican Susan Collins, opposes Chipman, pointing to his past comments that Collins said “demean law-abiding gun owners” and saying the relationship between ATF, the gun industry and sportsmen’s groups would be frayed if Chipman led the agency.

Sen. Jon Tester (D-Mont.) has also stressed for weeks that he is reviewing the nomination, and he hinted to reporters on Capitol Hill Tuesday that it could be withdrawn.

“I’m still reviewing it, and I’m not even sure it’s going to come up,” Tester told reporters.

Another notable holdout is Sen. Joe Manchin III (D-W.Va.), the perennial swing vote who remains an ally of gun rights advocates, despite his role in crafting an agreement eight years ago that expanded background-check requirements for firearms owners.

Chipman recently participated in a Zoom town hall hosted by Manchin that was virtually attended by West Virginia residents, with the conversation focused on gun rights, according to the White House official, who spoke on the condition of anonymity to disclose private meetings. The nominee has also met with the state’s Republican governor, Jim Justice, at Manchin’s request.

Senate Democratic leaders have publicly acknowledged that Chipman’s confirmation would be a difficult one, particularly since the position has been filled by a succession of acting chiefs after nominees from various presidents were unable to win Senate confirmation.

The only ATF director successfully approved by the Senate was Todd Jones, who was confirmed in 2013 under the Obama administration. He left the post in 2015.

“This is a highly controversial position. It has been left vacant because Republicans and Democrats couldn’t find someone to fill the slot,” Sen. Richard J. Durbin (D-Ill.), the chairman of the Judiciary Committee, said last week. “It’s no wonder that we’re having some difficulty with it.”

On Tuesday, Durbin — who is also the Senate Democrats’ whip, or chief vote-counter — acknowledged again that the votes are not there yet for Chipman to win confirmation.

“Still not ready for a vote,” Durbin said. He declined to identify which senators presented the biggest obstacles. “Let’s just say we’re working it.”

Chipman noted in his confirmation hearing in May that he is a gun owner and that the Supreme Court has recognized a constitutional right to own firearms, saying his primary objective as ATF director would be to enforce gun laws already on the books.

Still, Senate Minority Whip John Thune (S.D.), the No. 2 Senate Republican, said he believes all 50 GOP senators will oppose Chipman, whom Thune said has “just kind of an almost, in some ways, sort of open hostility toward people who are gun owners in this country.”