WASHINGTON — Senator Bob Casey was overdressed for the resistance. But he seems to have made it in time.
On the January evening of President Trump’s first executive order banning travel from several predominantly Muslim countries, Mr. Casey — an understated, conspicuously moderate Democrat from Pennsylvania — darted early from a Philadelphia Orchestra ball, still in his white tie and tails, to be at the airport where families were being detained.
When he got wind this month of the deportation of a Honduran mother and her 5-year-old son, the senator, whose voice rarely exceeds the decibel level of a yoga instructor, could be found on MSNBC, building to an unrecognizable semishout, after his hail of missives to the Trump administration went unheeded.
And when Mr. Trump’s firing of James B. Comey as F.B.I. director last week returned the Capitol to characteristic chaos, Mr. Casey was among the first to post on Twitter what several Trump critics had long been thinking: “This is Nixonian.” The message has been shared nearly 10,000 times.
Mr. Casey is not behaving like a senator approaching a re-election race next year in a state Mr. Trump carried, erasing any expectation that vulnerable Democrats would edge toward Mr. Trump en masse and distinguishing himself from some more reticent colleagues.
Nor is Mr. Casey behaving, according to some friends and supporters, entirely like himself — or, at least, the iteration they had come to expect during his even-tempered decade in Congress under presidents not named Trump.
Yet, as the anti-Trump movement continues, it has accommodated a leadership role for Mr. Casey, 57, the son of a governor from a suddenly-red state, initially elected to the Senate as an anti-abortion, pro-gun product of Scranton, Pa. — that irrepressible exporter of blue-collar political narratives for Bidens and Clintons and most any other candidate with a credible Rust Belt connection and a story to tell.
Of course, times change, and senators, too.
But Mr. Casey insists his higher gear has existed all along, suggesting that his circumstances have shifted far more than his legislative priorities, which have long skewed toward a familiar sort of Democratic Catholicism: programs for children, people with disabilities and other disadvantaged groups.
“We’re in a period of time where we’ve never been before,” Mr. Casey said in an interview at his office in the Capitol — once occupied by another noted Catholic Democrat in the Senate, John F. Kennedy. “I’ve been fighting these battles for years.”
Composure is central to the Casey political brand. There is a family joke about a stubborn mood ring given to Mr. Casey in the 1970s: It never changed colors.
Still, admirers say they can identify Mr. Casey’s recent spark.
“Trump has gotten his Irish up,” said Paul Begala, the Democratic strategist who first encountered Mr. Casey when his father, Robert P. Casey, ran successfully for Pennsylvania governor in 1986.
During off hours back then, Mr. Begala said, he would join the younger Mr. Casey for pickup basketball in Philadelphia. Swinging elbows were common. The senator has described his own skill-set as “blue-collar banger.”
For Mr. Casey, the Capitol’s most prominent anti-abortion Democrat, the more visible Trump-era profile also coincides with a national reckoning over whether opponents of abortion rights should have a place in the party’s future. Last month, Thomas E. Perez, the party’s newly elected chairman, said it was “not negotiable” for Democrats to “support a woman’s right to make her own choices about her body and her health.”
Other party leaders have countered that such a litmus test could doom Democrats to perpetual minority status. In private, many elected officials have invoked Mr. Casey as an example of the kind of figure the party would do well not to alienate.
The Casey family’s opposition to abortion is enshrined in a Supreme Court decision: Planned Parenthood v. Casey, which, in 1992, reaffirmed Roe v. Wade but upheld part of a Pennsylvania law regulating access to abortions during the elder Mr. Casey’s tenure. The former governor died in 2000.
During his time in the Senate, though, the younger Mr. Casey has become an ally of sorts for Planned Parenthood, fighting Republican efforts to defund the organization.
“I think our party is a much bigger tent than it was 10 or 15 years ago,” Mr. Casey said in his office, where a Pope Francis doll is perched beside his desk. He suggested that work on economic priorities for Democrats could transcend social issues.
As he seeks re-election next year, Mr. Casey has charted a different course than some fellow Democrats from states that Mr. Trump won, several of whom have trod more carefully. Generally, such Democrats, like Senators Joe Donnelly of Indiana and Heidi Heitkamp of North Dakota, represent far more right-leaning states.
Perhaps most notably, Mr. Casey was an early and forceful opponent of the nomination of Justice Neil M. Gorsuch to the Supreme Court, joining the Democratic filibuster without hesitation.
Pennsylvania Republicans say his choices will not go unnoticed.
“Even before the Trump presidency, he’s been moving consistently to the left, abandoning any pretense of being the man his father was, somebody who could reach across the aisle,” said Val DiGiorgio, the chairman of the state’s Republican Party, who predicted that Trump-voting Democrats in Western Pennsylvania would abandon Mr. Casey, too.
Mr. Casey’s possible challengers include Representative Lou Barletta, one of Mr. Trump’s earliest campaign supporters in Congress.
Edward G. Rendell, the former governor who won the job after defeating Mr. Casey in a 2002 primary, said Mr. Casey’s heightened outspokenness was not politically foolproof.
“You can say that Bob Casey is doing this at some political risk to himself,” Mr. Rendell said, adding that he believed Mr. Casey’s “evolution” in tone reflected a genuine anger at the Trump administration. “He always was a Democrat who ran well with moderates and even some reasonable conservatives. Is he throwing that away by being so vocal and emphatic on these issues? Well, maybe so.”
Mr. Casey’s office noted that of the 31 Pennsylvania counties he had visited since the election, 21 voted for Mr. Trump.
And his words have carried outsize weight in Democratic caucus meetings, where the party continues to grapple with how to recover its standing with working-class white voters.
“He’s one of my favorites,” Senator Chuck Schumer of New York, the minority leader, said of Mr. Casey, sounding like a teacher at a parents’ conference. “That quiet tone shouldn’t fool anybody.”
In all likelihood, though, Mr. Casey’s seat became safer with November’s result, allowing him to position himself against a sitting president rather than defend a third consecutive Democratic term in the White House.
Mr. Rendell predicted that Mr. Casey and Gov. Tom Wolf were the “only two Pennsylvania Democrats who will benefit from Donald Trump winning.”
Several colleagues and supporters rejected any suggestion that Mr. Casey had changed much in substance with the arrival of Mr. Trump.
“It’s the Bob Casey I’ve known for 10 years,” said Senator Sherrod Brown, Democrat of Ohio. “Trump hasn’t changed him. Trump has forced all of us to be more outspoken.”
At the very least, Mr. Casey, who became an ally of President Barack Obama after endorsing him over Hillary Clinton in 2008, seems to have established himself as a broadly reliable Democrat — one whom the party’s leading voices appear eager to embrace in public.
It was not always that way.
When Mr. Casey was weighing a Senate run before the 2006 elections, Mr. Rendell remembered, he received calls from Mr. Schumer and Mrs. Clinton, who was then a senator from New York. They asked if Mr. Rendell, then the governor, could help clear the primary field for Mr. Casey, he said. He obliged.
Then came the deluge of protests from abortion rights activists. Mr. Rendell called the senators back, asking if he could ease the pressure by spreading word that Mr. Casey had their support.
“They said, ‘We’ll get back to you,’” Mr. Rendell said, chuckling. “It’s now 10 years later. They’ve never gotten back to me.”