WASHINGTON — President Trump’s nominee to lead the Internal Revenue Service is a longtime tax lawyer who has specialized in defending people and companies against the tax agency in court.
On Thursday, Mr. Trump formally nominated Charles P. Rettig, a Beverly Hills, Calif., lawyer with the firm Hochman, Salkin, Rettig, Toscher & Perez, to succeed John A. Koskinen, the commissioner whose term ended in November.
If confirmed, Mr. Rettig faces a steep task in overseeing the agency as it begins enforcing the sprawling $1.5 trillion tax overhaul Mr. Trump signed into law last year amid a shrinking staff and budget. Since 2010, the I.R.S. budget has been cut by $900 million — or 17 percent, after adjusting for inflation — and its staff reduced by 21,000, or 23 percent.
The agency has also been under attack by Republicans, who have accused it of improperly targeting conservative groups seeking tax-exempt status, and by Mr. Trump. During the campaign, Mr. Trump, who has refused to release his personal tax returns, accused the agency of unjustly hounding him with audits year after year and said that paying as little as possible was “the American way.”
As commissioner, Mr. Rettig would oversee the audit of Mr. Trump’s personal tax returns. The president has in the past said he would only release the returns once they were no longer under audit.
Writing for Forbes in 2016, Mr. Rettig said he would not advise Mr. Trump, then a candidate, to release his tax returns while an audit was continuing. “Is there any legal impediment to Trump publicly releasing his tax returns? Absolutely not,” Mr. Rettig wrote. “Would any experienced tax lawyer representing Trump in an I.R.S. audit advise him to publicly release his tax returns during the audit? Absolutely not.”
Mr. Rettig went on to speculate that Mr. Trump’s returns “might actually be somewhat unremarkable but for the fact they are the returns of Donald Trump,” and that it was “unlikely an accurate overall financial picture will surface by simply reviewing his returns. He likely pays taxes at a lesser rate than many of us given the nature of his real estate and similar investments being subjected to lower tax rates than salaries earned by the rest of us.”
Mr. Rettig also mused whether Mr. Trump might be inflating his wealth. “He is likely worth far more than us,” Mr. Rettig wrote, “but may be worth far less than the approximately $10 billion he wants us to believe.”
Mr. Rettig has deep experience with disputes over the amount of taxes paid. He has defended clients against the I.R.S. in tax fraud investigations. He has twice led negotiations that resulted in settlements between the I.R.S. and Americans who had been improperly sheltering money overseas.
Last year, he sued the I.R.S. on behalf of a California bank holding company over a $1.5 million corporate tax bill that the company said was wrongly assessed. In 2014, he filed a similar suit on behalf of a California couple over $644,000 in tax penalties and interest that the couple said the I.R.S. had wrongly assessed.
After the release of the so-called Panama Papers in 2016, which detailed offshore tax sheltering by some of the world’s richest and most powerful people, Mr. Rettig wrote in a separate article for Forbes that “Anyone, located anywhere, who is potentially impacted by the public release of the Panama Papers should consider immediately contacting competent, experienced counsel.”
Mr. Rettig would bring more than 35 years of experience practicing tax law to the agency. He has overseen the I.R.S. Advisory Council and several professional organizations, including the taxation section of the State Bar of California.
Republicans expressed hope on Thursday that Mr. Rettig might provide a fresh start to their relationship with the I.R.S.
“It’s past time we restore Americans’ faith in this agency,” Senator Orrin G. Hatch, Republican of Utah and the chairman of the Finance Committee, said. “I look forward to learning more about Mr. Rettig and his qualifications to both achieve this goal and ensure an efficient implementation of the biggest tax rewrite in over 30 years.”