Mr. O’Rourke, who represented the city in Congress until the start of this year, said he would abandon the relatively traditional approach he has so far taken — with limited success — and largely detach his travel from a primary calendar that tethers most candidates to a handful of early-voting states like Iowa and New Hampshire.

Instead, Mr. O’Rourke said he would now plan his political activities around confronting Mr. Trump in direct and personal terms, and highlighting what Mr. O’Rourke views as the injustices of Mr. Trump’s administration. He intends to seek out immigrant-rich towns to campaign in, and to make gun control a central issue…

Beyond his imminent trip to Mississippi, Mr. O’Rourke said he envisioned campaign trips to visit Muslim communities that Mr. Trump had demonized, as well as people who were in jail and certain parts of the country that voted heavily for Mr. Trump.

“More than ever,” Mr. O’Rourke said, “we have to run a national campaign.”