The Senate's second-ranking Democrat suggested Sunday that Republicans are going after former secretary of state Hillary Clinton over last year's deadly attack in Benghazi with an eye on hurting her image ahead of the 2016 presidential campaign.

"Unfortunately, this has been caught up in the 2016 presidential campaign--this effort to go after Hillary Clinton," Sen. Dick Durbin (D-Ill.) said on CBS News's "Face the Nation." "The reason she wasn't interviewed was she didn't have any direct-line responsibility for the decisions that were made, but they want to bring her in because they think it's a good political show, and I think that's unfortunate."

Sen. Kelly Ayotte (R-N.H.) said she was "surprised" an independent Accountability Review Board did not probe Clinton "in detail." Former ambassador Thomas Pickering, who chaired the board, defended the decision.

"You had Secretary Clinton but you didn’t ask her any questions? And why not?" asked "Face the Nation" host Bob Schieffer.

"Because in fact, we knew where the responsibility rested," Pickering said. "She had already stated on a number of occasions, she accepted as a result of her job, the full responsibility. On the other hand, legislation setting up our board made it very clear that they didn’t want a situation in which a department or agency had accepted responsibility and then nobody looked at where the decisions were made."

GOP lawmakers have been increasingly focusing their investigation of the deadly attack on a diplomatic outpost last year on Clinton, who is widely viewed as the front-runner for the Democratic presidential nomination in 2016, should she run.

But House Oversight Committee Chairman Darrell Issa (R-Calif.) insists it is not about Clinton or President Obama.

"Hillary Clinton's not a target. President Obama is not a target," Issa said on NBC News's "Meet The Press." "The target is how did we fail three different ways."

The Sept. 11, 2012 attack that claimed the lives of four Americans, including Ambassador J. Christopher Stevens, returned to the forefront of the conversation in Washington last week during congressional hearings on the incident. Sen. John McCain (R-Ariz.) said Sunday on ABC News' "This Week With George Stephanopoulos" that the Obama administration's reaction to the attack amounted to a "cover-up."

Sen. Dianne Feinstein (D-Calif.) said on "Meet The Press" that the GOP focus on Clinton is concerning.

"My concern is that when Hillary Clinton's is mentioned 32 times in the hearing that a point of the hearing is to discredit the [former] secretary of state, who has very high popularity and may well be a candidate for president," she said.