Can the El Paso Shooter Make a Political Defense ... And Be Gagged in Court for Inciting Racial Violence?

There is speculation that the El Paso shooter surrendered easily in order to have his day in court to incite racial violence.

Can a judge gag a defendant in court during the trial? Yes. 
Bobby Seale was a Black Panther, part of a group that had a history of black nationalist violence. After he disrupted his trial in the “Chicago Eight” case (for inciting a riot during the 1968 Democratic convention), Judge Abbe Hoffman famously bound and gagged him.

The Supreme Court ruled on this extraordinary judicial power in Illinois v. Allen (1970), stating a judge may order an unruly defendant bound and gagged if the person has been repeatedly warned against disrupting the court.

Given the “repeated warning” rule, the El Paso shooter could poison minds by the time he was gagged.

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Perhaps a better question is whether the shooter can raise a defense of protecting his race from replacement by Hispanics.

This is not out of the question—and it may lead prosecutors to agree to a plea deal to save his life and keep him off the witness stand. 

Lawyers call this a shadow defense. It is used to get juries to “nullify” their legal instructions and sympathize with the defendant on non-legal (here, political grounds).

In Texas courts, unanimous juries are needed to convict. If one juror sympathizes with the shooter, there would be no conviction.

In sum: This is not a slam-dunk case for prosecutors, given the prevalence of bigotry and xenophobia in America ... and rules that fulsomely protect defendants. 
Prosecutors may decide not to proceed with a hate crime case if they believe it will only backfire by inspiring more mass shootings.