Fetterman's vote to advance the nomination of Mullin, an Oklahoma Republican, to a floor vote from the Senate Homeland Security Committee was crucial as Kentucky Republican Senator Rand Paul broke ranks from his party and voted against his nomination. Mullin advanced following the 8-7 vote.
Trump picked Mullin to lead the department beginning at the end of March to replace Kristi Noem, whose tenure at DHS has been riddled with controversy amid the administration's heightened immigration enforcement. If confirmed, Mullin will lead one of the most prominent agencies responsible for immigration and border security, a key cornerstone of Trump's political agenda.
Fetterman addressed his vote in a statement Thursday morning.
"In January, I called on the president to fire Noem -- and he did. I truly approached the confirmation of my colleague and friend, Senator Mullin, with an open-mind. We need a leader at DHS. We must reopen DHS. My AYE is rooted in a strong committed, constructive working relationship with Senator Mullin for our nation's security," he wrote.
His support for Mullin drew outrage from Democrats. Representative Brendan Boyle, who represents a Philadelphia-based congressional seat, was among those calling out Fetterman over the vote.
"Once again Sen Fetterman shows why he is Trump's favorite Democrat. He needs to go," Boyle wrote on X.
Former Representative Connor Lamb, who lost to Fetterman in the 2022 Senate primary, responded, "Did people think this vigilante was voting to protect their rights? Come on."
Shannon Watts, founder of Moms Demand Action, responded on X, writing, "Can Pennsylvanians sue him for impersonating a Democrat?"
"Fetterman, the DINO," wrote progressive commentator Mehdi Hasan, using the acronym for "Democrat in Name Only."
Fetterman has established himself as a more centrist member of the Senate who has been willing to cross party lines in the past, often drawing Democratic ire. However, he has dismissed questions about whether he would be willing to switch parties to become a Republican, touting his support for abortion rights and the LGBTQ+ community as key causes on which he remains aligned with his party.
Fetterman earlier urged Trump to fire Noem amid DHS backlash over the administration's approach to immigration enforcement.
"Americans have died. She is betraying DHS's core mission and trashing your border security legacy. DO NOT make the mistake President Biden made for not firing a grossly incompetent DHS Secretary," he said in a statement in January.
Fetterman is up for reelection in 2028 but has not said whether he intends to run. Democrats have called for him to face a primary challenge if he chooses to seek another term.
A recent poll from Quinnipiac University revealed that a slim majority of Pennsylvanians approve of Fetterman's job performance. Forty-six percent of respondents gave him positive marks while 40 percent disapproved.
However, the poll found a low approval rating -- 22 percent -- among Democrats, 62 percent of whom disapprove of Fetterman.
His approval is bolstered by Republicans, 73 percent of whom approve of him and 18 percent disapprove.
The poll surveyed 836 registered voters from February 19-23, 2026, with a margin of error of plus or minus 4.7 percentage points.
Political scientist Don Moynihan wrote on Bluesky: "Fetterman literally giving Mullin the key vote to be appointed will be something I suspect will be an issue if he runs for re-election."
The Pennsylvania Working Families Party wrote on X: "Despicable. John Fetterman was the deciding vote to advance Trump's nominee to lead ICE. We deserve better."
Tré Easton, a former Fetterman aide, wrote on X: "By my count (and I have been counting), I believe this is the first time his vote was actually decisive -- as in something would've failed to advance without it. Helluva moment to do that when ICE is trying to build a 520,000 sq ft detention warehouse in his state."
Mullin's nomination will now head to a floor vote. If no other Republicans oppose his nomination, he would be confirmed because the GOP holds a 53-47 majority in the chamber.