WASHINGTON (AP) — 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 as head of the Pentagon.

"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. Hagel's wife, Lilibet, backed Obama, while the former Nebraska lawmaker stayed on the sidelines.

In a statement on 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.

A handful of Senate Republicans have announced that they will vote against Hagel's nomination, while other GOP lawmakers remain noncommittal. Hagel is tentatively scheduled to meet with Sen. Susan Collins, R-Maine, who has not said how she will vote, when lawmakers return the week of Jan. 21.

While Democrats hold a 55-45 advantage in the Senate, and Hagel would need a simple majority to win confirmation, he would need 60 votes if Republicans try to filibuster his nomination.

Although Hagel served in the Senate for two terms and was a member of the Foreign Relations and Intelligence panels, several Republicans and Democrats say he didn't build many close relationships and tended to chart his own course.

Of the 45 Senate Republicans, just 25 served with Hagel. Among the newer, more moderate members, Hagel is certain to face a tough time persuading Sen. Mark Kirk, R-Ill., who has been at the forefront in pushing for unilateral sanctions against Iran and is a fierce proponent of Israel.

Santorum, who unsuccessfully sought the GOP presidential nomination last year, recently led a successful fight to defeat a U.N. treaty on the disabled, convincing several Senate Republicans to oppose ratification. He overcame such vital treaty supporters as McCain and former Sen. Bob Dole.

Supporters of Hagel's nomination are also lobbying on his behalf. Former Secretary of State Colin Powell will appear on NBC's "Meet the Press" on Sunday to argue for the nominee.