WASHINGTON - Chad Wolf was sworn in Wednesday as the new acting secretary of the Department of Homeland Security, becoming the fifth person to hold the top job under President Donald Trump, a period of unusually high leadership turnover at the nation's largest domestic security agency.

Wolf will be joined at DHS headquarters by Ken Cuccinelli, the acting director of U.S. Citizenship and Immigration Services, who will move into the acting deputy secretary role, according to two administration officials familiar with the plans.

Cuccinelli had been Trump's preferred choice for the top job, but Senate Majority Leader Mitch McConnell, R-Ky., dislikes Cuccinelli, and senior Republican senators have indicated that they will not confirm him for a permanent job.

Wolf replaces Kevin McAleenan, who assumed the acting DHS chief role when Trump removed Kirstjen Nielsen in April. McAleenan received the president's praise but was never nominated for the job, and he submitted his resignation Oct. 11.

The White House then struggled to find a candidate to replace McAleenan, because so many of the other top positions at DHS remain vacant or have interim leaders who lack Senate confirmation.

Wolf cleared a key hurdle Wednesday when senators voted 54 to 41 to confirm him for a different job, DHS undersecretary for strategy, plans and policy. That vote, largely along party lines, allowed him to move into the top role at DHS, which has 240,000 employees and a $50 billion budget.

How long Wolf and Cuccinelli will be in the acting roles is unclear. White House officials say Trump does not plan to nominate Wolf for the permanent position, and some of the immigration restrictionists who back the president have criticized Wolf's prior lobbying work on behalf of foreign companies that sought employment visas.

Cuccinelli, the former Virginia attorney general and conservative activist, made bitter enemies on Capitol Hill when he ran a political action committee that challenged several GOP incumbents. But placing Cuccinelli alongside Wolf allows the administration to install him in an even more central immigration enforcement role, giving the White House a Trump surrogate who is fully at ease on social media and in front of television cameras.

One official familiar with the plan said both Wolf and Cuccinelli would work on border-related issues, a reflection of the president's view of DHS as an immigration enforcement agency. Cuccinelli is expected to make the transition to DHS headquarters in the next few days, according to one official who spoke on the condition of anonymity to describe the internal planning.

Wolf, 43, began his DHS career with the Transportation Security Administration and then worked for the lobbying and consulting firm Wexler & Walker from 2005 to 2017. He was chief of staff to Nielsen during last year's failed "Zero Tolerance" border crackdown that led to the separation of at least 2,700 children from their parents. Democrats who voted against Wolf cited his role in the episode.

Despite that record, and Trump immigration adviser Stephen Miller's support for Wolf, some hard-liners view him as insufficiently committed to Trump's border policies. His profile at DHS has been that of a cautious, more managerial figure, and not a conservative ideologue.

Though DHS includes such disparate agencies as the Coast Guard, Secret Service and the Federal Emergency Management Agency, Trump's attention to the department has centered almost entirely on its immigration and border-related components: U.S. Customs and Border Protection, U.S. Immigration and Customs Enforcement, and U.S. Citizenship and Immigration Services.

Wolf, like his predecessors, will inherit a migration dynamic at the southern border of the United States that remains in flux. During fiscal 2019, which ended Sept. 30, authorities took nearly 1 million migrants into custody along the U.S.-Mexico border, the highest total since 2007.

After peaking at more than 144,000 detentions in May, arrests have fallen by two-thirds, the result of an enforcement crackdown by the Mexican government as well as a suite of experimental Trump administration deterrent policies applied to asylum seekers. They include the "Migrant Protection Protocols" that require asylum seekers to return to Mexico while their claims are processed in U.S. courts, leaving many to wait for months on end in squalid camps and dangerous Mexican border towns.

Many of the Trump policies are facing legal challenges in federal court, and DHS officials say a decision that takes away enforcement tools could trigger a new migration surge.

Last week, the Republican chairman and ranking Democrat of the Senate Homeland Security and Governmental Affairs Committee sent Trump a letter urging him to pick a DHS nominee and to fill the many other vacancies at the department as soon as possible.

"The widespread use of temporary leadership - individuals who, though perhaps qualified, do not serve with the imprimatur of having been confirmed by the Senate - makes it more difficult for the Department to achieve its long-term strategic objectives," wrote Sen. Ron Johnson, R-Wis., and Sen. Gary Peters, D-Mich., warning of the "dangers of pervasive vacancies to government accountability and national security."

"The American people deserve leaders who will ensure stability and accountability to the Department," they told the president.

Trump has repeatedly said he prefers having "acting" leaders in top leadership roles because he believes it gives him more flexibility to fire them.

DHS was established after the Sept. 11, 2001, attacks to safeguard the country from another terrorist plot, and maintaining stable leadership at the department was a high priority for Presidents George W. Bush and Barack Obama.

During the two terms Bush was in office, he had three different DHS chiefs. Obama also had three DHS secretaries during the course of his eight years.

This article was written by Nick Miroff, a reporter for The Washington Post.