Sanders missed more votes than any other senator during the first quarter of 2016, skipping 37 of the Senate's 38 roll call votes, according to GovTrack.

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It's the first time Sanders has led the Senate in missed votes since announcing his presidential bid last year, and he has greatly surpassed previous candidates' records.

Many of the votes the two senators missed were either procedural or situations where their votes wouldn't have changed the outcome. But they've also skipped voting on legislation to combat opioid addiction — a major issue that has been in the spotlight during the campaign.

Clinton's camp took a swing at Sanders earlier this year after he missed the vote on new sanctions against North Korea.

"It is unfortunate that yet again, Sen. Sanders has shown a lack of interest in vital national security issues," Clinton campaign spokesman Jesse Ferguson said after the vote.

Sanders said he was "necessarily absent" during the North Korea vote, but that the increased sanctions were "absolutely essential" to ending North Korea's nuclear program.

Cruz's campaign has also defended his ability to balance his job in the Senate with the presidential race. Being on the campaign trail didn't stop Cruz from temporarily blocking the Senate from confirming State Department nominees, and he also temporarily put a hold on the Senate's energy reform bill and Flint, Mich., aid package.

He's not the only Republican who has faced questions about his voting record. Rubio, who ended his campaign earlier this year, caught frequent flak for missing votes.

The Florida Republican has defended his record, saying he votes when he believes he can make a difference in the outcome or on important pieces of legislation. Rubio is retiring at the end of his current term.

Rubio missed more votes than any other senator in 2015. He also comes in third — behind Sanders and Cruz — for the most votes missed during the first quarter of 2016.

He returned to the Senate earlier this month and voted to hold classified ads site Backpage.com and its chief executive in contempt.

The vote marked Rubio's fourth roll call vote so far this year.