WASHINGTON — Republican opposition to Chuck Hagel as the next defense secretary swelled from inside and outside the Senate Thursday as a former Senate colleague launched a campaign to block his nomination.

Rick Santorum, the former Pennsylvania senator who served for a decade with Hagel, said his grassroots organization, Patriot Voices, will lead a nationwide campaign to stop Hagel from succeeding Leon Panetta.

“While I respect Sen. Hagel’s service to America, I cannot stand by and support his nomination,” Santorum said in a statement.

“His anti-Israel, pro-Iran mindset makes him uniquely unqualified to serve as our Defense secretary.”

Hagel, nominated Monday by President Barack Obama, has faced criticism that he is soft on Iran and weak in his support for Israel. Pushing back in private meetings with defense officials, he has told them this week that he backs strong international sanctions against Iran and believes all options, including military action, should be on the table.

He also has reached out to members of the Senate and sought to explain his views on national security.

One of his first calls was on Monday to Sen. John McCain, a fellow Vietnam War veteran who worked closely with Hagel on a number of issues but split with him over the Iraq war.

The relationship between the two hit a rough patch in 2008, when Hagel declined to endorse McCain in the presidential race.

In a statement Monday, McCain said Hagel served the nation with honor in Vietnam. But McCain expressed serious concerns about some of the positions Hagel has taken on national security issues.