Former Virginia Attorney General Ken Cuccinelli (R) on Monday began his new job as acting director of U.S. Citizenship and Immigration Services (USCIS), a controversial appointment that could set up a showdown between the White House and Senate Republicans.

President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE tapped Cuccinelli to lead the agency, which is tasked with administering the nation’s legal immigration system, at a time when he is seeking to crack down on illegal migration and make it tougher for immigrants to obtain benefits.

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“Our nation has the most generous legal immigration system in the world and we must zealously safeguard its promise for those who lawfully come here,” Cuccinelli said in a statement distributed by the Department of Homeland Security (DHS). “I look forward to working with the men and women of USCIS to ensure our legal immigration system operates effectively and efficiently while deterring fraud and protecting the American people.”

Cuccinelli’s appointment was weeks in the making, with Trump eyeing the fellow immigration hard-liner for a top role at DHS.

He will replace Lee Francis Cissna, who was forced out as USCIS chief late last month amid a rolling shakeup at the department that was engineered by top White House policy adviser Stephen Miller Stephen MillerSpecial counsel investigating DeVos for potential Hatch Act violation: report Trump confirms another White House staffer tested positive for COVID-19 Biden pick creates furor, underscoring bitterness over Obama immigration policy MORE.

USCIS did not explain the legal basis for Cuccinelli’s appointment. Under the Federal Vacancies Reform Act, an acting official must be the agency’s top deputy, another Senate-confirmed official or a senior agency official who had been in their job for at least 90 days prior to a vacancy.

A DHS spokesperson said “Mr. Cuccinelli’s appointment is in accordance with the Federal Vacancies Reform Act,” but did not elaborate and referred further questions to the White House, which did not immediately respond to a request for comment.

The White House has not formally nominated him as director.

Cuccinelli, however, could face a rocky road to confirmation should Trump nominate him to lead USCIS on a permanent basis.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellTrump 'no longer angry' at Romney because of Supreme Court stance On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline The Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power MORE (R-Ky.) has long opposed the former Virginia attorney general, dating back to the Cuccinelli-led Senate Conservatives Fund’s effort to oust the top Senate Republican in 2014. McConnell spoke out against Cuccinelli in April when his onetime foe was rumored to be in the running to lead DHS.

"He’s spent a fair amount of his career attacking Republicans in the Senate, so it strikes me as an odd position for him to put himself in to seek Senate confirmation,” Sen. John Cornyn John CornynQuinnipiac polls show Trump leading Biden in Texas, deadlocked race in Ohio The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight MORE (R-Texas) said last week, according to Politico. “It’s unlikely he’s going to be confirmed if he is nominated.”

Others expressed concern that Cuccinelli’s appointment signals the Trump administration’s intent to transform USCIS from an agency responsible for distributing immigration benefits, such as visas and green cards, to one that is focused on enforcement.

Cuccinelli is a longtime critic of birthright citizenship, a right conferred by the 14th Amendment to the Constitution, and has called for limiting in-state tuition at public universities to U.S. citizens and legal permanent residents.

The head of a union representing USCIS employees said late last month that Cuccinelli’s selection would spell "the end of legal immigration as it currently exists.”

“It has become clear that the goal of this administration is to end immigration all together. How better to do that then by appointing as the leader of USCIS someone who knows nothing about immigration, adjustment of status or naturalization, and whose sole purpose is to destroy the agency that grants these benefits,” Danielle Spooner, president of the American Federation of Government Employees, told CNN.

Updated at 2:40 p.m.