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 semi-shout, 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.