Senate Republicans Wednesday pushed President Obama’s pick to lead the Department of Homeland Security to crystallize his stance on immigration reform, demanding a greater commitment to border security.

Obama DHS nominee Jeh Johnson, the Pentagon’s former defense counsel, waded into debates about immigration reform, surveillance and interrogation techniques, drone attacks and the current threat of al Qaeda, showcasing the litany of challenges awaiting the lawyer if confirmed to lead the sprawling federal agency.

But GOP leaders, who have raised questions about Johnson’s experience and his close ties to the president, also suggested that the former Pentagon official would be one of the few Obama nominees of late to win approval from the upper chamber.

Republican lawmakers, however, used the Senate Homeland Security and Governmental Affairs Committee hearing to wage a proxy fight over how to better secure the border.

Sen. John McCain, R-Ariz., who at one point predicted that Johnson’s confirmation “will be very smooth,” also ripped Obama’s nominee for not guaranteeing that the U.S. would reach 90 percent-effective control of the border.

Johnson said he was inclined to cooperate with Republican requests but stopped short of issuing a concrete metric for effective policing of the border.

McCain countered that he could not support Johnson's nomination until he received such a guarantee.

Johnson in general said he was for “comprehensive, common-sense immigration reform” that included an “earned path to citizenship.”

An immigration reform package that passed the Senate earlier this year has stalled in the House. Speaker John Boehner, R-Ohio, said Wednesday that Republicans have no plans to bring the Senate bill to conference.

Obama’s DHS pick was somewhat of a surprise choice, as Johnson does not have a lengthy track record on immigration issues, which have become a central focus of the agency, in addition to combating terrorism.

Johnson is most well-known for helping craft the administration’s legal rationale for an unprecedented drone campaign overseas and ending the military’s ban on openly gay service members.

Johnson was also asked to weigh in on the raging Washington debate about the tradeoffs between privacy and national security, a clash stoked by a series of leaks detailing National Security Agency surveillance programs.

"Do you think the Fourth Amendment applies to my Visa purchases?” Sen. Rand Paul, R-Ky., asked Johnson.

The DHS nominee was careful not to make any statements that contradicted the administration’s stance on controversial surveillance tactics.

For his part, Johnson said his primary goal was to fill senior-level DHS vacancies — roughly 40 percent of which remain vacant.

“As I speak, the department of government charged with the vital mission of homeland security has no secretary, no deputy secretary and a number of other senior positions are vacant,” Johnson said. “If confirmed as secretary, my immediate priority… will be to work with the White House and the Senate to fill the remainder of these key leadership positions.”

Republicans recently blocked Obama’s pick to lead the Federal Housing Finance Agency, Rep. Mel Watt, D-N.C., and multiple judicial nominees for the D.C. Circuit Court of Appeals. But GOP officials on Wednesday appeared to take a different approach with Johnson.

“I believe you will be confirmed," Sen. Tom Coburn, R-Okla., said, adding that Johnson needed to be more transparent than his predecessor, Janet Napolitano, who resigned earlier this year to oversee the California higher-education system.