The traditionally collegial U.S. Senate was never a natural fit for Frank Lautenberg, the wealthy New Jersey businessman whose headstrong, CEO style could rankle.
But the five-term senator, who died early Monday at age 89, managed to serve as a passionate and able advocate for a tight collection of causes, from gun control and public health to Israel and mass transit.
Lautenberg was the force behind banning smoking on commercial airline flights and raising the national drinking age, and he was instrumental in early and ongoing gun control efforts, including efforts to restrict purchases of assault weapons.
He had announced in February that he would not seek re-election in 2014.
President Obama, in a midday statement, lauded the late senator and founder of the payroll services behemoth ADP as a public servant who "improved the lives of countless Americans with his commitment to our nation's health and safety, from improving our public transportation to protecting citizens from gun violence to ensuring that members of our military and their families get the care they deserve."
Lautenberg's death, due to complications from viral pneumonia, leaves a Senate vacancy that's now in the hands of New Jersey's popular Republican Gov. Chris Christie.
Christie, expected to win re-election easily in November, appears to have a variety of options, from calling a special Senate election this year to appointing a successor for the remaining 1 1/2 years of Lautenberg's term.
A wild card? Christie could appoint himself, but that scenario appears far-fetched to politicos in New Jersey and beyond.
"He would do violence to his brand," says Michael Murphy, a one-time New Jersey Democratic gubernatorial candidate and former county prosecutor.
Says Senate expert Ross Baker, a professor at New Jersey's Rutgers University: "That's not for him."
While eulogies for Lautenberg were still being written, a short list of potential Republican appointees began to take shape.
Those mentioned include state Sen. Joe Kyrillos, who last year lost a U.S. Senate bid to incumbent Democratic Sen. Robert Menendez; state Sen. Tom Kean Jr., son of former New Jersey Gov. Tom Kean; state Assembly Leader Jon Bramnick; and three-term U.S. Rep. Leonard Lance.
New Jersey hasn't had a Republican senator since Nicholas Brady was appointed to serve out disgraced Democratic Sen. Harrison Williams' term in 1982.
The last elected New Jersey Republican senator was Clifford Case, who left office in 1979.
Whatever the succession scenario Christie settles on, the chosen one will likely face Newark Mayor Cory Booker, expected to be the Democratic nominee for Senate.
In his statement, Obama paid tribute to Lautenberg, from his humble beginnings as a son of working-class immigrants and service in the Army, to his founding of ADP and long service in the Senate.
Baker, the Rutgers expert, characterized Lautenberg, who served on the powerful Appropriations Committee, as "more an executive than an actual senator, a self-made man who came to the Senate with a perspective that didn't particularly fit well."
"But he was a dependable, liberal Democrat who pursued with considerable success the issues he cared deeply about: gun control, public transportation, public health and a restrained foreign policy," Baker says.
His net worth is estimated at between $57 million and $117 million, according to the Center for Responsive Politics. Between 2007 and 2012, Lautenberg donated $654,600 to his own campaigns.
Lautenberg's Senate service had a brief interruption. He retired in 2000, in part, Baker says, because he thought he'd face a very tough GOP opponent — either then-Gov. Christine Todd Whitman or former Gov. Kean. But he got back in the mix in 2002, when his longtime nemesis, Democratic Sen. Robert Torricelli, abruptly ended his re-election effort amid a campaign contribution scandal.
Lautenberg replaced Torricelli as the Democratic candidate just six weeks before the election, and won a return to the Senate.
"Frank was a middle-bencher in the Senate," says Murphy, the New Jersey Democrat and lobbyist. "But he was passionate about public policy issues that he really engaged in."
In the mid-1990s, when Murphy was a county prosecutor, he and Lautenberg became close when the senator personally intervened with King Hussein of Jordan to ensure the return of two American children, kidnapped by their Jordanian father, to New Jersey.
"It was illustrative of Frank's passion," Murphy said. "He cared about the little people."
Lautenberg had battled cancer, and he also recently waged a public battle with Booker, the rising star who appeared a bit too anxious to nudge out the old Senate veteran.
In January, Lautenberg suggested that Booker might need a "spanking" for being "disrespectful" for moving toward a run while an incumbent still held the seat.
"I have four children. I love each one of them," he said at the time. "I can't tell you that one of them wasn't occasionally disrespectful, so I gave them a spanking and everything was OK." A month later, Lautenberg announced he would not seek re-election.
Here's what he told the Newark Star-Ledger:
"I am not announcing the end of anything. I am announcing the beginning of a two-year mission to pass new gun safety laws, protect children from toxic chemicals and create more opportunities for working families in New Jersey. While I may not be seeking re-election, there is plenty of work to do before the end of this term and I'm going to keep fighting as hard as ever for the people of New Jersey in the U.S. Senate."