For Democrats, Bob Casey said, the uptick in grassroots energy is “inspiring them to do more and think about things they probably wouldn’t be doing.” | AP Photo

PHILADELPHIA — Sen. Bob Casey is known as easygoing and open to bipartisanship. But as the liberal base grows increasingly restive, the Pennsylvania Democrat is discovering his inner firebrand.

Casey never raised his voice before the 700 constituents who turned out Sunday for his first in a series of town hall meetings — but he didn’t need to in order to send a message to the largely liberal audience. From fights over health care to the Supreme Court, Casey signaled that he’s allying with the millions of protesters who have flooded the streets to resist President Donald Trump.

Trump narrowly carried Pennsylvania in November, and Senate Republicans are eagerly targeting Casey in the 2018 midterms. But Casey embraced the anti-Trump sentiment in the crowd at the University of Pennsylvania, which represents the base of support he’ll need to turn out if he hopes win a third term and turn the Keystone State back to blue.

Casey’s approach suggests even vulnerable Senate Democrats who hail from swing states that backed Trump are prepared to align with the party’s rising grassroots.

The mostly friendly group was pleased with his promise to “fight like hell” against the GOP’s Obamacare repeal legislation. And the crowd cheered his vow to support multiple investigations into Trump’s ties to Russia.

When one man said he wanted to see Trump resign, Casey gently responded, “I don’t think that’s going to happen,” before adding, “But here’s what I do know — this kind of activism and engagement is having an impact already on both parties.”

For Democrats, Casey said, the uptick in grassroots energy is “inspiring them to do more and think about things they probably wouldn’t be doing.”

The 56-year-old, who followed his father Gov. Robert Casey Sr. into politics, may have been talking about himself.

“He always has been a little bit bland in the past, since he’s been in office,” said Colleen Frank, who traveled to Casey’s Philadelphia town hall with fellow members of the Upper Buxmont Rising activist group. “But he is stepping up now. He’s much more animated.”

Casey didn’t commit to supporting a filibuster of Trump’s Supreme Court nominee, Judge Neil Gorsuch. But he outlined “real, significant, major concerns” with Gorsuch and added that even if his review of the judge’s record goes well, he’s still worried the nominee would move the high court in a “more corporate direction.”

Frank didn’t mind that Casey stopped short of pledging a filibuster…