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WASHINGTON - A senior immigration official in the George W. Bush administration is defending President Obama's embattled choice for a top homeland security post, saying the nominee's willingness to make tough decisions means coworkers "might be unhappy with him and want to embarrass him" with unfair allegations.

Alejandro Mayorkas, the current director of U.S. Citizenship and Immigration Services, who has been nominated to be deputy secretary for the Department of Homeland Security, is one of several immigration officials under investigation by the department's internal watchdog over complaints of alleged impropriety.

But in a letter Monday to the Senate Homeland Security and Governmental Affairs Committee, which must approve the nomination before it can proceed, the former chief counsel and acting director of USCIS says such complaints come "with the territory of leadership" and "need to be viewed in full context."

"Managing and coordinating the operations and policy of the Department of Homeland Security is a monstrous role. Not many people are equipped to do it well," Robert Divine wrote in the letter, obtained by ABC News. "I believe that Alejandro Mayorkas is one of those people."

The department's inspector general is specifically looking into allegations raised last year that Mayorkas intervened on behalf of an electric car company co-founded by Democratic operative Terry McAuliffe, now a candidate for Virginia governor. Frustrated by the slow pace of processing visa requests for investors, McAuliffe appealed to senior USCIS officials for help.

During his Senate confirmation hearing last month, Mayorkas "unequivocally" denied any wrongdoing.

"I have never, ever in my career exercised undue influence to [change] the outcome of a case," an often-somber Mayorkas told lawmakers, his voice halting at times. "I have never based my decisions on who brings a case, but rather upon the facts and the law."

In his letter of support Monday, Divine praised Mayorkas' "probing intellectual curiosity, his incisive sparring among proponents of divergent views," and "his respect for rigorous process."

The allegations against Mayorkas first became public last month after the DHS inspector general's office summarized its investigation in an email to lawmakers' staff. In that much-publicized email, the inspector general's office said it has yet to find any evidence of criminal conduct.

Days later, after Republicans boycotted Mayorkas' confirmation hearing, the inspector general's office quietly sent lawmakers a "follow-up" email saying investigators had "not made any preliminary findings as to the substance of any allegation made against Mr. Mayorkas."

Mayorkas has yet to be interviewed in the probe, but he has said he is eager to weigh in.

The top Republican on the Senate committee, Tom Coburn, R-Okla., recently said the allegations, "if true, may directly apply" to Mayorkas' fitness to serve as DHS deputy secretary, which is the No. 2 post at the department.

At the center of the allegations against Mayorkas is a U.S. program that allows well-funded foreigners who are willing to invest in businesses in the United States to obtain a green card if their investments are likely to create U.S. jobs.

First created by Congress in 1990, the EB-5 program has long been a target of criticism and controversy, and a top Republican has recently raised national security concerns over it, noting that a visa was recently sought by a Chinese telecommunications firm under congressional investigation last year for alleged ties to Chinese intelligence services. The firm has previously denied those charges.

Nevertheless, as USCIS director, Mayorkas "saw that interpretational and operational difficulties were impeding the full use of the program" for "the benefit of U.S. workers," and he "made deep policy dives into the program," which included meetings with stakeholders, developers and attorneys "to understand the challenges and develop balanced solutions," Divine said in his letter Monday.

"The result, at last, has been a very recent 'clearing of the decks' of longstanding" applications, but "whenever a leader takes decisive action, some people disagree, and I am not surprised that some in the agency might be unhappy with him and want to embarrass him," Divine wrote.

A former top USCIS official under the Bush administration disagreed with Divine's assessment.

"[Mayorkas] overruled his own process," the former official said, speaking on the condition of anonymity and drawing conclusions from press reports and materials released by Grassley. "When you overrule your own process, that raises a lot of questions."

During his confirmation hearing, Mayorkas told lawmakers the McAuliffe-related matter rose to his attention only because it involved a series of narrow, complex legal and policy questions. At one point, Mayorkas met with McAuliffe to hear his grievances, namely that applications for visas to foreign investors were taking so long to be processed.

USCIS officials originally rejected his company's application.

"I heard Mr. McAuliffe's complaints and I moved on with my work," Mayorkas told lawmakers during his confirmation hearing. "I do remember returning to [my] office and complaining about the fact that I had to hear complaints."

Mayorkas said it is "noteworthy" that such complaints continue to this day, so the people he allegedly improperly helped are still not satisfied.

Grassley, however, has said that "contrary to the impression left" by such testimony, the one meeting with McAuliffe "was clearly not the extent" of Mayorkas' interaction on the matter with firms associated with McAuliffe and other high profile names.

Grassley has cited several emails between USCIS officials and the firms, including an August 2011 email asking Mayorkas for a "check on the status" of some applications. Mayorkas did not respond, one of his deputies did.

Prior to leading USCIS, Mayorkas was a federal prosecutor for 12 years, notably becoming the U.S. attorney for the Central District of California. He also was a private lawyer in Los Angeles.

Though the former USCIS official speaking on the condition of anonymity was critical of Mayorkas' alleged actions, he called the current situation "very unfortunate."

Mayorkas "genuinely is a good" and "decent guy" who means well, the former official said.

Coburn's office declined to comment for this article.

Divine, who wrote the letter Monday supporting Mayorkas, was chief counsel to USCIS from July 2004 until November 2006. He served as acting director and then acting deputy director of USCIS from July 2005 until July 2006. Federal databases show he has repeatedly donated money to Republican campaigns.

ABC's Chris Good and Shushannah Walshe contributed to this report.