In the wake of a string of horrific mass shootings, one Republican state lawmaker wants the California Republican Party to do more to denounce white nationalism and racism.

Assemblyman Chad Mayes, R-Yucca Valley (San Bernardino County), said in a tweet this week that he will propose a formal resolution at the party’s next state convention this fall.

“Republicans must reject racism, xenophobia, nationalism, and white supremacy. The belief that one group of people is superior to another is fundamentally un-American,” Mayes tweeted, with the hashtag #epluribusunum.

Mayes didn’t specifically mention President Trump in his tweet, but there’s little mystery about the intended subject of his comments. The lawmaker has regularly criticized the president and last month called Trump’s attacks on four House members who are women of color “beyond unacceptable.”

However, Mayes’ effort likely will face opposition from conservative Republican delegates. He’s often faced criticism from within his party and was ousted as Assembly GOP leader in 2017, after he supported exetending the state’s cap-and-trade emissions program.

Among them is Corrin Rankin, a delegate from Redwood City, who said she would support a “sincere resolution” but questions Mayes’ motives. Rankin, a black woman, said she expects delegates could see it as an attempt to criticize Trump.

“My concern is that given how unpopular Assemblyman Mayes is with many delegates because of the gas tax deal he helped broker, that such a resolution would fail based on those merits alone,” she said.

In a follow-up tweet, Mayes added that while he thinks the state party has done good work, “we must be loud and concise that this ideology has no place and is not tolerated in Lincoln’s party.”

Mayes’ tweet came after massacres in Gilroy; El Paso, Texas; and Dayton, Ohio, left 34 people dead and injured dozens more. The accused El Paso shooter, who killed 22 people in a Walmart, is believed to have written an anti-immigrant manifesto about a “Hispanic invasion of Texas.”

Critics of President Trump have likened some words in the shooter’s apparent manifesto to Trump’s language about an immigrant “invasion” on the southern border.

While Mayes didn’t specifically mention El Paso or Trump in his tweet, he retweeted a series of comments accusing Republicans of enabling white nationalism.

Among the posts Mayes shared was a tweet from Stanislaus County Supervisor Kristin Olsen, a Republican and former state assemblywoman, that singled out Trump.

“When a leader of a nation carelessly (or deliberately) uses words to rally anger & hostility that leads to growing nationalism, racism, white supremacy, & hatred, it is incumbent upon other leaders to speak up and condemn such words & behavior,” she tweeted.

Mayes is expected to introduce his resolution at the state GOP’s fall convention, Sept. 6-8, in Indian Wells (Riverside County).

The lawmaker’s efforts to reshape the Republican Party’s image are nothing new. Last year, Mayes helped form New Way California, a GOP organization that advocates for moderate stances on issues like immigration and climate change.

Joe Justin, Mayes’ spokesman, said the lawmaker wouldn’t comment about his tweet or release a copy of the proposed resolution now, saying, “The tweet should speak for itself until we know more.”

Dustin Gardiner is a San Francisco Chronicle staff writer. Email: dustin.gardiner@sfchronicle.com Twitter: @dustingardiner