WASHINGTON — The Department of Homeland Security is beginning to address white supremacist terrorism as a primary security threat, breaking with a decade of flagging attention after bigoted mass shooters from New Zealand to Texas took the lives of nearly 100 people in the last six months.

In a little-noticed strategy document published last month to guide law enforcement on emerging threats and in recent public appearances by Kevin K. McAleenan, the acting secretary of homeland security, the department is trying to project a new vigilance about violent white nationalism, beating back criticism that the agency has spent a decade playing down the issue.

“I would like to take this opportunity to be direct and unambiguous in addressing a major issue of our time. In our modern age, the continuation of racially based violent extremism, particularly violent white supremacy, is an abhorrent affront to the nation,” Mr. McAleenan said during an address last month, describing white nationalism as one of the most dangerous threats to the United States.