Mike Huckabee is going perhaps the furthest of his Republican presidential rivals in slamming the National Security Agency (NSA) for reportedly sweeping up lawmakers' conversations while snooping on Israel.
"The real issue is whether or not the president was aware that members of Congress were being tapped," the former Arkansas governor said during an interview with Fox News on Wednesday night. 
"If that's the case, and the president was aware of it, Sandra, I believe that this is not only unconstitutional, I believe this is an impeachable offense," Huckabee told host Sandra Smith.
"Richard Nixon resigned for less than this. I don't think we can just gloss this over. This is a serious issue when one branch of government spies on another branch," Huckabee added. 
Wall Street Journal report published Tuesday said that the NSA had continued snooping on Israeli Prime Minister Benjamin Netanyahu and other U.S. allies despite Obama saying two years ago it would curb the practice.
The breadth of the NSA's surveillance of foreign leaders and their aides was publicly released in documents disclosed by former NSA contractor Edward Snowden in 2013. 
The Journal reported that while collecting the communications of Netanyahu and his aides, the NSA also picked up conversations with lawmakers. The administration decided to let the NSA decide what information to share, a U.S. official told the newspaper.
House Intelligence Committee Chairman David Nunes (R-Calif.) said Wednesday that his panel would look into whether the U.S. intelligence community swept up communication between Israeli officials and members of Congress. 
Huckabee said Wednesday that if the executive branch spied on members of Congress, Obama "should either resign or the Congress ought to have the guts to go after him."
Other GOP candidates have also criticized the NSA's actions.
Ben Carson called the report of NSA snooping "truly disgraceful," and Sen. Rand Paul (Ky.) used it to criticize excessive government surveillance. Sen. Marco Rubio (Fla.), a member of the Senate Intelligence Committee, acknowledged "concerns" about the U.S. snooping on its ally Israel.