WASHINGTON — Outgoing Housing Secretary Julián Castro was a regular presence on the campaign trail for Hillary Clinton, but in recent weeks he has taken his criticism of President-elect Donald Trump to the next level.

Trump is so corrupt that if Democrats controlled even one chamber of Congress, he'd be done before he even got started. #2018 — Julián Castro (@JulianCastro) December 27, 2016

So Trump-- the shadiest, most corrupt guy to take the Oval Office, will have no strong federal checks and balances. Has to change. #2018 — Julián Castro (@JulianCastro) December 27, 2016

The fiery tweets have put the San Antonio Democrat back in the spotlight on his way out the door of President Barack Obama's Cabinet, attracting media attention around the country and beyond, and suggesting he may assume a new role as a political attack dog.

This growing scandal now seems like it may get too big for Republicans to control, try as they will. https://t.co/hlJmqPLBHP — Julián Castro (@JulianCastro) December 14, 2016

Here's the thing about Donald Trump-- he thinks he's above you and your family. He thinks the people work for him, not the other way around. — Julián Castro (@JulianCastro) December 10, 2016

Donald Trump doesn't respect you, your family or our country enough to dedicate himself full time to the job of being President. — Julián Castro (@JulianCastro) December 9, 2016

Castro was critical of Trump’s campaign throughout 2016, predicting up until election night that the Republican candidate’s rhetoric would awaken the “sleeping giant” of Latino voters. But he has taken a decidedly more hostile tone since the election.

The inclusion of #2018 in Castro’s recent tweets has reignited chatter about what might be next for the former San Antonio mayor as he looks ahead to the next election cycle.

Castro has been mostly quiet about his plans since the election, but allies say he will be on the lookout for a swift return to electoral politics when the opportunity arises. He currently plans to return home to San Antonio after leaving office and finish an autobiography that has been in the works for several years.

His twin brother, Democratic Rep. Joaquin Castro of San Antonio, has been openly considering a Senate run to challenge Republican Sen. Ted Cruz. He, too, has been an outspoken Trump critic on social media since the election.

....And for paintings of you hanging at your hotels. https://t.co/Ayo5Bgd2bN — Joaquin Castro (@JoaquinCastrotx) December 27, 2016

Prediction: After major falling out characteristic of his relationships, Trump will have a worst relationship w/ Putin than any recent Pres. — Joaquin Castro (@JoaquinCastrotx) December 22, 2016

During the campaign, Julián Castro's political advocacy led to an inadvertent violation of the Hatch Act — which prohibits federal employees from engaging in political activity while in their official capacity — when he talked about both department business and his support of Clinton in an interview with Katie Couric.