WASHINGTON — When President Trump came to Pennsylvania earlier this month, he slapped a nickname on Bob Casey: "Sleepin' Bob."

It was the most high-profile example of an attack that Republicans have long leveled at the Democratic senator with a famous family name: that Casey is a nice guy, but a nonentity in Washington.

As he completes his 12th year in the Senate and seeks another term, the mild-mannered Casey is not readily associated with leadership on any national issue, or with bold proposals that push the boundaries of debate.

"You're never going to see Casey taking the podium surrounded by a bunch of other senators announcing a big piece of legislation," said Kristin Kanthak, a political scientist at the University of Pittsburgh. "He's more incremental."

By contrast, fellow Pennsylvania Sen. Pat Toomey has made himself into one of Republicans' key leaders on economic issues. He played a central role writing the GOP's tax and health care bills last year, and made national waves with a proposal to expand background checks for gun purchases.

In New Jersey Sen. Bob Menendez is the top Democrat on foreign policy and a national voice on immigration, while Sen. Cory Booker is a potential presidential candidate pushing criminal sentencing reform.

Casey argues that dozens of laws he pushed have provided meaningful benefits for his constituents, even if they don't generate headlines or boost his profile.

"In a partisan time when very few bills get passed, I've got a very strong record, not just passing legislation, but working with Republicans to get it done," he said. "I'm not too concerned with how people view my so-called national record. My first obligation is to the people of Pennsylvania."

While senators like Toomey and Menendez have attained broader influence by making themselves authorities on specific national issues, Casey doesn't have a distinct lane. He tends to work on a patchwork of less-visible concerns.

"You could think of Casey as being a solid singles hitter and Toomey being more of a guy who swings for the fences," said Kanthak.

"He's lived his entire political career not being defined by anything," said Mark Dion, a Republican consultant who has worked on Pennsylvania races.

Perceptions of Casey are partly shaped by his low-key demeanor and monotone speaking style. To many in Pennsylvania's political world, his most distinctive traits are his father, the former governor, and his unusually kind and polite manner.

Republicans in 2012 dubbed him "Senator Zero" — though Casey still won big. Now he is facing a challenge from Republican U.S. Rep. Lou Barletta.

Casey's bills usually take on narrow topics and fall within a safe consensus. He often promotes them by citing letters from Pennsylvanians.

By his own measure, Casey's biggest achievement is a bill that allows families to set up tax-free savings accounts, similar to 401(k)s, to pay for the long-term needs of children with disabilities. As of May there were 1,306 so-called ABLE accounts in Pennsylvania, according to Casey's office. Advocates for people with disabilities hailed the bill as a "landmark" when it passed.

Casey also highlighted his leading role on a measure requiring colleges to develop written policies on how they handle sexual assault and domestic violence. He was the lead Senate Democrat on a bill to increase funding for crumbling county bridges, a problem in rural Pennsylvania, and when the Affordable Care Act passed he added a $250 million fund to help young women who are mothers or pregnant complete their educations and obtain support services. The idea was to reduce abortions, and grants have been given in 22 states.

More recently Casey was the top Senate Democrat on a bill that renewed and updated the law that provides $1.2 billion for career and technical education programs, aiming to help people seeking alternatives to four-year colleges.

The measure, like much of Casey's work, wasn't controversial or groundbreaking. It cleared the Senate by a voice vote and kept the gears turning for a popular program.

"He picks and chooses sort of low-hanging fruit among a range of issues," said Jack Johannes, a political science professor at Villanova University. "To some extent I give him credit for that, because there aren't that many senators who do that kind of stuff, and that sort of stuff needs to be done."

While Toomey, a former businessman, has a risk-taking entrepreneurial streak, Casey is more "managerial," said Alan Novak, a former Republican state chairman in Pennsylvania.

"It's impossible to get real excited about him, but it's impossible to dislike him," Novak said.

Casey argued that he is "a national leader" on children's issues, though none of the five independent analysts contacted for this story named him as a standout on any topic.

At the same time, they said Casey's modest public profile isn't unusual.

As party leaders have exercised more control over debate and legislating has slowed to a crawl, rank-and-file lawmakers have fewer ways to make an imprint, said Wendy Schiller, a Brown University political scientist who has studied the Senate.

Senators from a given state often have contrasting approaches, Schiller said: When one focuses on the big picture, the other frequently has a more parochial style.

It also takes coordination and effort to pass even simple measures in a chamber where one senator can stop them, said Sarah Binder, who researches Congress at George Washington University.

Sen. Kamala Harris (D., Calif.), who recently campaigned with Casey, said, "We've got to respect the people who do the hard work everyday without ego … and that's Bob Casey."

Casey aides say 44 bills he has introduced or been a prime cosponsor on have become law, often by being folded into broader legislation (a common practice). Some were adopted in full, others in part, and the list ranges widely in scope and ambition.

Among them: a bill to create a commission to celebrate the country's 250th anniversary. It passed by unanimous consent.