Mr. Kushner has also been interviewed by Robert S. Mueller III, the special counsel, though it is not known whether any potential actions relevant to the Russia inquiry are part of the delay in finishing Mr. Kushner’s background check.
Abbe D. Lowell, Mr. Kushner’s lawyer, said in a statement that “it is not uncommon for this process to take this long in a new administration (some taking as long as two years)” and that there are “a dozen or more people at Mr. Kushner’s level whose process is delayed like his.”
Questions about the security clearance process at the White House have become more urgent after the scandal surrounding Mr. Porter and the still-unanswered questions about when the president’s aides knew about the abuse allegations against him. On Monday, Sarah Huckabee Sanders, the White House press secretary, again refused to provide a detailed explanation.
“I can’t get into the specifics,” Ms. Sanders said in response to questions about what Donald F. McGahn II, the White House counsel, knew about the Porter allegations and when he knew it.
Ms. Sanders referred questions about the security clearance process — and why Mr. Porter was allowed to continue working at the White House for so long despite the abuse accusations — to the F.B.I. and the intelligence agencies, saying they are the ones that handle the background checks and the granting of permission to handle classified information.
“It’s up to those same law enforcement and intelligence agencies to determine if changes need to be made to their process,” she said. “If changes are thought to be made, that would be made by the law enforcement and intel communities that run that process, not the White House. But that’s something that could be looked at, certainly, in light of this.”
The F.B.I. had no comment on Monday.
The finger-pointing has frustrated Democratic members of Congress, who have pushed to gain visibility into the security clearance procedure at the White House — and the possible holdups for the clearances of a number of staff members — but have found their efforts largely stymied by Republicans.
Representative Trey Gowdy, Republican of South Carolina and the chairman of the House Oversight Committee, has refused to ask the White House for any information about security clearances or for a formal briefing on the matter, Democrats on the panel said Monday. He has also refused to allow the committee to vote on three subpoenas proposed by Democrats, including one on interim clearances.
Mr. Gowdy’s office did not respond to a request for comment.
In a letter sent last week, Representative Elijah E. Cummings of Maryland, the top Democrat on the committee, pressed Mr. Gowdy to more aggressively tackle the security clearance issue, saying that news of Mr. Porter’s problems had given the matter renewed urgency.
“If you had agreed to any of our previous requests for information on these matters, the White House would have been required to answer key questions about why Mr. Porter was denied a final security clearance, who at the White House was aware of this information, and how Mr. Porter was allowed to remain in his position,” Mr. Cummings wrote.
“Instead, because of your multiple refusals, we did not find out about any of these issues until they were reported in the press,” he added. “In this and many, many other areas, it appears that the Oversight Committee has constructed a wall around the White House in order to prevent any credible oversight whatsoever.”
The process for allowing White House officials to work with secret, classified material typically begins when West Wing employees are subjected to a brief background check and are granted interim clearance. A full background check by the F.B.I. and, in some cases, the C.I.A., then begins.
Officials with previous administrations said it is not uncommon for the full background checks to take as long as eight months or a year, in part because of a long backlog in vetting the backgrounds of people needing clearance across the federal government. For example, Defense Department officials estimated recently that nearly 100,000 people who work with military contractors hold interim clearances while waiting for their full background checks.
Background checks take about a year for Pentagon employees and six to seven months for prospective C.I.A. or National Security Agency workers, a senior American official said.
But several former White House officials said it is also common for administration officials to encourage the investigators to give top priority to senior presidential aides like Mr. Porter and Mr. Kushner. In urgent cases, like the appointment of a cabinet secretary, permanent security clearances can be granted within weeks, the former officials said.
Mr. Porter told Mr. McGahn in January 2017 that there could be what he described as false allegations against him, according to two people briefed on the situation. In June, the F.B.I. told Mr. McGahn that allegations of domestic abuse had surfaced, but Mr. McGahn encouraged the bureau to keep investigating.
In November, the bureau informed Mr. McGahn that Mr. Porter was not likely to succeed in getting a permanent clearance, according to one person briefed on the case, but Mr. McGahn requested that the F.B.I. complete its investigation and come back to the White House with a final determination about the allegations.
In Mr. Kushner’s case, a final decision about a permanent security clearance is also still pending.
Mr. Lowell said that it should not be surprising that someone with “the extent of his holdings, travels and lengthy submissions” would require a lengthy background check. But he took issue with the suggestion that the delays somehow affect Mr. Kushner’s ability to do his job at the White House.
“This is just the latest,” he said, “in unnamed sources quoting secondhand hearsay concerning Mr. Kushner that, like the others, will be shown to be untrue.”