Hillary Clinton's campaign and Bernie Sanders are exchanging Twitter burns

Former Secretary of State Hillary Clinton's presidential campaign and Sen. Bernie Sanders (I-Vermont) have exchanged a number of Twitter blows in recent days.

Sanders in particular has launched some zingers. On Wednesday, the Vermont senator tweeted a photo of a handwritten note Clinton had apparently written to him back in 1993. The post was a thinly veiled jab at Clinton for ratcheting up attacks against his healthcare-policy agenda in the race.

"To Bernie Sanders with thanks for your commitment to real healthcare access for all Americans," the Clinton note read.

Both Clinton and Sanders have been publicly sparring as Clinton, the Democratic front-runner, has seen her lead over Sanders melt away both nationally and in the first two voting states. Clinton has also launched broadsides against Sanders for backing a bill that protected gun manufacturers.

On Monday, Sanders dug up a two-year-old tweet from John Podesta, Clinton's campaign chair, that backed a single-payer healthcare system. Sanders, a big supporter of single-payer healthcare, playfully offered Podesta a job on his campaign:

Podesta responded by torching Sanders' approach to healthcare policy.

On Wednesday, he continued the criticism:

At the end of last week, Sanders highlighted the fact that Clinton criticized President Barack Obama's position on guns during her 2008 primary campaign against him:

Meanwhile, the official Clinton campaign account "The Briefing" has also been hammering away at Sanders: