A growing number of Republicans on Wednesday pressed for a delay in considering the nomination of a senior homeland security official who is under investigation for allegations that he improperly aided a politically connected firm in gaining U.S. visas for foreign workers.

The Republicans, led by ranking member Sen. Tom Coburn of Oklahoma, wanted to delay a hearing scheduled Thursday for immigration official Alejandro Mayorkas, who has been nominated to be second-in-command at the Department of Homeland Security.

Sen. Charles E. Grassley (R-Iowa) also released 40 pages of documents and national security questions about Mayorkas and the programs under his jurisdiction.

“We believe it is neither appropriate nor fair to consider the nomination of an individual under investigation,” said a letter from Republicans sent to Committee Chairman Thomas R. Carper (D-Del.).

But Carper aides have said the hearing would proceed as planned, and the White House signaled its continued support for the nominee Wednesday.

Presidential counsel Kathryn H. Ruemmler wrote in a letter to Carper that Mayorkas, director of U.S. Citizenship and Immigration Services, underwent FBI background checks and White House vetting before his nomination was put forward.

“We have no concerns about his suitability for this important position,” she wrote.

The feuding over the nomination erupted this week following news that the DHS inspector general was investigating whether Mayorkas improperly intervened in the EB-5 visa program, which permits foreign nationals to enter the United States if they agree to invest $500,000 to $1 million to create U.S. jobs.

Mayorkas’s assistance was sought in January by Anthony Rodham of Gulf Coast Funds Management — the brother of former secretary of state Hillary Rodham Clinton — on behalf of GreenTech Automotive, a firm co-founded by Virginia gubernatorial candidate Terry McAuliffe (D).

McAuliffe’s Republican opponent, Virginia Attorney General Ken Cuccinelli II, has attacked him for GreenTech’s ties to the probe.

Grassley said in a letter to Carper and Coburn on Wednesday that he has learned from whistleblowers that other federal agencies have raised national security concerns about the EB-5 program, which is under Mayorkas’s jurisdiction.

“I’ve received information and documents from whistleblowers over the last few days, which demonstrate that the Director was directly involved in expediting EB-5 applications before the proper security checks,” Grassley wrote. “The FBI has expressed serious national security concerns with foreign investors involved in some of the EB-5 projects that had moved forward.”

Grassley also said the Office of the Inspector General is investigating Mayorkas’s conduct in several areas beyond the approval of the visa applications. Grassley said the problems include “allegations of conflicts of interest, misuse of position, mismanagement of the EB-5 program, and an appearance of impropriety.”

His letter specifically cited questions raised about applications made on behalf of GreenTech.

A Carper spokeswoman, Emily Spain, said late Tuesday that the hearing would go forward as planned, in part because the forum would provide an opportunity for questioning the nominee in public and under oath.

“This type of public and transparent back and forth should be encouraged, not stifled,” Spain said. “While many of the committee’s members differ in their view of this nominee, Chairman Carper believes that it is critical that the Committee allow the normal confirmation process to move forward by holding tomorrow’s nomination hearing.”