Originally Published: 26 JUL 21 18:35 ET

Updated: 26 JUL 21 22:28 ET

By Manu Raju, Betsy Klein and Maegan Vazquez, CNN

(CNN) -- The White House and Democratic Senate leaders are trying to salvage President Joe Biden's pick to lead the Bureau of Alcohol, Tobacco, Firearms and Explosives, but it's not clear whether enough senators will get behind a nominee who has faced intense opposition from gun rights activists.

The White House hasn't given up on David Chipman's nomination to lead the bureau, according to two senior administration officials. One source said the administration was still hoping Senate leaders would schedule a vote in order to put the members on record.

But Senate Majority Leader Chuck Schumer has long been leery about holding votes on matters that won't get the support of all 50 of his members -- and a single Democratic defection would derail Chipman's nomination.

At issue are a handful of moderate members, in particular independent Sen. Angus King of Maine and Democratic Sen. Joe Manchin of West Virginia, who have yet to commit to supporting the nomination. All Republicans are expected to vote against Chipman, despite the nominee's previous efforts to get some moderate Republicans -- including Republican Sen. Lisa Murkowski of Alaska -- on board.

King and Manchin signaled earlier this month that they hadn't been sold on Chipman's nomination.

"The issue is whether he's the right guy for the job," King, who caucuses with Democrats, told CNN. "My question is whether he can be an effective director. I haven't decided yet."

When asked he'd back Chipman, Manchin said, "We're still working on it," adding that they had a "very candid conversation."

Chipman would be the first Senate-confirmed director of the agency since 2015 and just the second in its history. Biden nominated him to the role in April.

Chipman is a former career ATF employee, having worked for the agency for 25 years. He was also a gun control activist, working as a senior adviser to Everytown for Gun Safety and Giffords -- the organization started by former Congresswoman Gabrielle Giffords, who was shot at an event in her Arizona district in 2011.

The National Rifle Association opposes Chipman's nomination, but he's garnered the endorsement of several law enforcement groups. Complicating the vote for Democrats is that Chipman has faced a series of tough blows from hunting and sportsman groups, like Ducks Unlimited, that have argued that his views on an assault weapons ban are too extreme for the job.

"This is the first time I have seen such a broad array of sporting groups, conservation groups like Ducks Unlimited come out in opposition to a nominee. That shows how divisive he is," said GOP Sen. Susan Collins of Maine earlier this month. Collins was one of those GOP senators who decided against Chipman, a sign of stiffening Republican opposition.

The resistance to Chipman's appointment comes amid a rise in mass shootings in the US and as Democrats have been trying for months to craft gun control-related legislation that could overcome GOP pushback. The House approved two bills to expand background checks on firearm sales, including one to do so on private and commercial transactions, but they lack the support of moderate Senate Democrats like Manchin and Sen. Jon Tester of Montana.

Earlier this month, Chipman was taking part in private one-on-one meetings with reluctant senators to assure them he respects the Second Amendment, including planned meetings with Murkowski and King. A meeting with pro-gun groups in West Virginia and GOP Gov. Jim Justice at Manchin's request was also on the books.

Senate Judiciary Chairman Dick Durbin of Illinois, the majority whip, told CNN on Monday that they were still "working" the nomination in an attempt to secure the votes.

The White House said in a statement that it continues to support Chipman's nomination.

"David Chipman's 25 years of distinguished service as an ATF agent uniquely equips him to reinvigorate the Bureau's critical mission, including combatting gun trafficking and keeping firearms out of the hands of criminals. Confronting the epidemic of gun violence head-on is a cornerstone of the President's gun crime strategy, and he looks forward to the Senate quickly voting on Chipman's nomination as ATF Director," said White House spokesman Michael Gwin.