Trump nominated Nielson because he believes that he, like other Trump nominees, would be a rubber stamp for his radical agenda, which threatens women, workers, LGBTQ people, immigrants, and people of color.
From the Alliance for Justice:
Nielson’s motion reflected the rhetoric of certain right-wing groups. The American Family Association said that it was “extremely problematic that Judge Walker is a practicing homosexual himself. He should have recused himself from this case, because his judgment is clearly compromised by his own sexual proclivity.”
Conservative activist Tony Perkins specifically said that Judge Walker’s decision was compromised by the fact he is “openly homosexual.”
Nielson argued, in attempting to vacate the judgment, that only if Judge Walker had “unequivocally disavowed any interest in marrying his partner could the parties and the public be confident that he did not have a direct personal interest in the outcome[.]”
Kamala Harris, then California Attorney General, filed a brief opposing the motion. She wrote, “[j]ust as every single one of the attempts to disqualify judges on the basis of their race, gender, or religious affiliation has been rejected by other courts, this Court should similarly reject DefendantIntervenors’ effort to disqualify Judge Walker based on his sexual orientation.”
When Mr. Nielson served as a political appointee in the Ashcroft Justice Department, he politicized the hiring process and undermined the credibility of the nation’s chief law enforcement agency. According to a 2008 report written by the Justice Department’s Inspector General and Office of Professional Responsibility, Mr. Nielson was part of a working group of political appointees that wrested control of the hiring process away from career attorneys. The report stated: “As our report describes, in 2002 the Honors Program [the Department’s program for hiring junior attorneys] and SLIP [Summer Law Intern Program] hiring process was fundamentally changed by an Attorney General’s Working Group to enable the Department’s senior leadership to have more input into the selection of candidates. Prior to that time, career employees within each component administered the interview and selection process for the Honors Program and SLIP.” 
Moreover, according to the report, Mr. Nielson appears to have served on a four-person screening committee in 2002 that discriminated against job applicants who had either worked for civil rights or liberal public interest groups, or for Democratic officials. The screening committee discriminated against job applicants who had affiliations with dozens of different civil rights and public interest organizations including 13 that are members of The Leadership Conference coalition: American Civil Liberties Union, Amnesty International, Brennan Center for Justice, Center for Reproductive Rights, Human Rights Campaign, Lambda Legal, Lawyers’ Committee for Civil Rights Under Law, Mexican American Legal Defense and Educational Fund, NAACP Legal Defense and Educational Fund, Inc., National Consumer Law Center, National Organization for Women, Planned Parenthood, and Sierra Club.
After career officials recommended approximately 900 candidates for interviews for the elite Honors Program, the four-person screening committee pared down the list to 600. The report analyzed the backgrounds of the roughly 300 deselected candidates, and the results were extremely troubling. Of the 100 applicants determined to be “liberal” based on their organizational affiliations, 80 were deselected and denied interviews. Of the 46 applicants determined to be “conservative” based on affiliations, only four were deselected. That is, 80 percent of perceived liberal applicants for the Honors Program were deselected, but only 9 percent of conservative applicants. Among the most academically qualified applicants, the report determined that 83 percent of liberal applicants were deselected but not a single conservative was deselected. The same politicization occurred for the prestigious summer intern program during the time Mr. Nielson likely served on the screening committee: 84 percent of liberal SLIP applicants were deselected (68 of 81) for interviews, while only 3 percent of conservatives were deselected (1 of 29). Similar disturbing disparities were found to exist for applicants who had worked for Democratic elected officials versus Republicans, and for applicants who were members of the American Constitution Society versus the Federalist Society.
Mr. Nielson was not disciplined for his actions because he had left the Justice Department by the time of the IG/OPR report, but when testifying about political appointees involved in a similar hiring scandal, Mr. Fine testified that “people did leave the Department so they cannot be disciplined by the Department, but we have recommended that they never get a job with the Department again, hopefully never with the Federal Government again.” We agree and believe the same applies to Mr. Nielson.