U.S. President Barack Obama is expected Monday to nominate former Republican Senator Chuck Hagel to be the country's next defense secretary.

Prominent Republicans signaled Sunday that the 66-year-old Hagel would likely face a tough confirmation process, with a focus on questions about his stances on Israel and Iran.

Hagel has opposed unilateral sanctions against Iran, while some lawmakers have questioned his level of support for Israel.

Senator Lindsey Graham said in an interview Sunday that Hagel is a “controversial choice,” and called his potential selection an “in-your-face nomination.”

Senate Minority Leader Mitch McConnell said Hagel will be given fair consideration by the Senate, which must approve the president's pick.

Hagel is a Vietnam War veteran who became a critic of the U.S. operation in Iraq. He left the Senate in 2009 and told The Financial Times in 2011 that the Pentagon was “bloated” and needed to be “pared down.”

If confirmed, Hagel would become the third defense secretary under President Obama, following Robert Gates and current Secretary Leon Panetta.