One day before former Pentagon lawyer Jeh Johnson gets grilled by senators during his confirmation hearing to become the next Homeland Security secretary, the Democrat and two Republicans who previously held the post threw their unanimous support his way on Tuesday.

"Mr. Johnson's abilities and experiences will not only continue the important effort the Department of Homeland Security is charged with, but build on the progress made over the last 10 years to combat an ever-evolving threat, and make our communities more resilient," former secretaries Tom Ridge, Michael Chertoff and Janet Napolitano said in a letter to leaders of the Senate Homeland Security and Government Affairs Committee.

For most of Obama's first term, Johnson served as the Defense Department's general counsel, reviewing and approving military operations before they were sent to the Defense Secretary and president for final decisions.

In their letter Tuesday, the three former secretaries said such experience has given Johnson "a perspective that will benefit him as he implements a key element of DHS' mission, combating terrorism here at home."

Johnson has been outspoken in his views on fighting terrorism overseas, including support for drone attacks targeting elusive terrorism targets.

But Johnson has remained largely silent on other matters that intersect with Homeland Security responsibilities, including countering homegrown radicalization, enforcing the nation's immigration laws, responding to natural disasters, and effectively sharing threat information with local law enforcement.

The three former secretaries wrote that Johnson's "service at the highest levels of the U.S. Department of Defense, the largest government agency in the world, provided him a keen understanding of and the logistics required to successfully execute large-scale, operational missions of varying complexity and purpose."

The letter from Napolitano, Ridge and Chertoff comes more than two weeks after a number of other top-level law enforcement officials sent their own letters of support. Among those who sent letters was former Attorney General Michael Mukasey, a frequent critic of some within the Obama administration.

"I have no hesitation in supporting his nomination," wrote Mukasey, who got to know Johnson decades ago when Johnson was a federal prosecutor in New York and Mukasey was a judge there. "[Johnson] will bring to DHS not only experience but also a frame of mind that should be a source of assurance to anyone concerned with the security of this country."

But not all Republicans are happy with Johnson's nomination.

"I have grave concerns," Sen. John Cornyn, R-Texas, said in a statement last month. "Rather than selecting someone who knows the unique dynamics of our southern border, President Obama has tapped one of his former New York fundraisers. We need someone who knows how to secure the border, not dial for dollars."

Sen. Lindsey Graham, R-S.C., has threatened to block Johnson's nomination if the Obama administration does not answer his questions about the terrorist attack in Benghazi, Libya, last year that left four Americans dead, including the U.S. ambassador there.

Neither Graham nor Cornyn is a member of the Senate committee presiding over Johnson's confirmation.

The Department of Homeland Security has been without a Senate-confirmed leader for two months.

Johnson's confirmation hearing gets underway Wednesday morning.

Most recently, Johnson was a litigator and civil criminal trial lawyer in New York with the firm Paul Weiss, where he's worked off and on for nearly three decades.