WASHINGTON \u2014 Temporary status is a seemingly permanent condition of the Trump administration. The resignation of Kirstjen Nielsen as Homeland Security secretary on Sunday means that another cabinet officer who reports directly to President Trump will have the word \u201cacting\u201d next to the official title at a major department of government. Interim secretaries are also in place at the Departments of Defense and of the Interior, and at the Office of Management and Budget, the Small Business Administration and ambassador\u2019s office at the United Nations. Mick Mulvaney, Mr. Trump\u2019s chief of staff, is also serving in an acting capacity. \u201cI like acting. It gives me more flexibility. Do you understand that?\u201d Mr. Trump told reporters in January before departing to Camp David. \u201cI like acting. So we have a few that are acting. We have a great, great Cabinet.\u201d But there are concerns about having men and women in such high-level jobs without having been subjected to Senate confirmation for those posts. Leaving cabinet secretaries unconfirmed in their roles could give the president even more leverage over them, or could leave them without full authority in the job. \u201cThe Senate grappled with this question in the very first Congress when it ordered George Washington to send nominations to the Hill at a reasonable pace,\u201d said Paul Light, a professor of public service at New York University who has written extensively on the appointment process. \u201cSenators rightly worried that presidents might use acting appointees to evade oversight and institutional prerogatives. Yet, we haven\u2019t heard a word from the Senate on the Trump administration\u2019s abuse of its acting authority.\u201d Justice Clarence Thomas argued in a concurring opinion in a 2017 case that there was seemingly no constitutional basis for having \u201cacting\u201d cabinet members and that there needed to be limits on a president\u2019s power to fill the highest positions without Senate confirmation. The constitution requires that the president obtain \u201cthe Advice and Consent of the Senate\u201d before appointing cabinet officials. Legal scholars also questioned the president\u2019s power to use the \u201cacting\u201d authority in replacing Ms. Nielsen. \u201cTo me, the real difference is avoiding Senate confirmation \u2014 either because the individuals he wants running these agencies can\u2019t be confirmed even by a Republican-controlled Senate, or because he\u2019s worried about the kinds of questions they\u2019d have to answer and or concessions they\u2019d have to make in order to be confirmed,\u201d said Stephen Vladeck, a law professor at the University of Texas. \u201cEither way, that\u2019s an alarming argument for bypassing a Senate controlled by his own party.\u201d Mr. Trump announced in a tweet on Sunday that Kevin McAleenan, 47, the commissioner of United States Customs and Border Protection, will be Ms. Nielsen\u2019s replacement, as \u201cacting\u201d Homeland Security secretary at a time when the president has said that there is a security crisis at the border with Mexico because of a flood of migrants trying to gain entry into the United States. Some acting officials like Patrick M. Shanahan, the acting defense secretary who had been deputy defense secretary, have been confirmed by the Senate but not in the post in which they currently serve. David Bernhardt, the acting Interior secretary, had already been confirmed as the department\u2019s deputy and is undergoing confirmation by the Senate for the top job. Mr. Trump had long said he was satisfied with the performance of Ms. Nielsen. He expressed similar optimism for her acting successor. \u201cI have confidence that Kevin will do a great job!\u201d Mr. Trump said in his tweet. No matter who Mr. Trump would have selected, it will take time to get fully operational. Government agencies, Mr. Light noted, must \u201cwade through piles of internal paperwork assigning responsibilities and signature authorities to one acting after another.\u201d Career government officials \u201cwonder when vacancies will be filled, how long the actings will stay, and whether they\u2019ve got the right name on the organization chart to move forward on major issues such as the Max 8 groundings,\u201d he said, referring to the airliner involved in two fatal crashes. Oversight by ethics and accountability agencies within the government is also constrained. . \u201cThe result is a fundamental collapse in ordinary accountability,\u201d Mr. Light said. In her resignation letter, Ms. Nielsen said she hoped the incoming secretary would \u201chave the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America\u2019s borders and which have contributed to discord in our nation\u2019s discourse.\u201d But the support of Congress may be difficult to obtain, notably among Democrats who have said Mr. Trump is usurping the role of a coequal branch of government. \u201cWithout confirmation hearings, there\u2019s less of a chance for the Senate to meaningfully play a role in identifying who will be in charge of these critical government functions, and thus less accountability for episodes like Acting Secretary Shanahan\u2019s ties to Boeing,\u201d Mr. Vladeck said. \u201cSecond, an acting secretary may have the same control over his or her own agency as someone who is Senate confirmed, but traditionally, Senate-confirmed officers have more pull in any interagency conflict, and so it has the effect of skewing the authority of different agencies toward those with permanent heads.\u201d What is more, Mr. Trump has relentlessly said that there is a \u201ccrisis\u201d on the border, yet the two cabinet departments most directly responsible for dealing with the situation, Homeland Security and Defense, do not have permanent leaders. \u201cIt\u2019s striking how many of the secretaries of the largest departments of this government are acting at this point,\u201d Senator Christopher Coons, Democrat of Delaware, said Monday on CNN.