Clinton does have one fairly easy way to accomplish the move to the left. Clinton could follow the always popularly discussed but rarely used strategy of creating a "dream ticket" and choosing Sanders as her running mate.
Clinton is no stranger to the "dream ticket" scenario. In 2008, there was a lot of talk about Barack Obama choosing Clinton as his vice president in order to appeal to her own then-embittered base. That obviously didn't happen, and it didn't happen on the other side of the aisle in 2012, when Mitt Romney passed over the runner-up Rick Santorum in order to choose a candidate who didn't run in the primaries.
In fact, it is relatively rare that a presidential contender chooses the runner-up as a vice presidential selection. Since John F. Kennedy created the controversial Boston-Austin Dream Ticket with then-Senate Majority Leader Lyndon Johnson in 1960, there has been only two other runner-ups selected to run as VPs: John Edwards in 2004, and George H.W. Bush in 1980.
Edwards ran a campaign that accentuated the positive – he spoke about his background and personal history and didn't burn many bridges by attacking John Kerry. Bush got the nod after Reagan had very unusual high-profile VP flirtation with former President Gerald Ford that failed. There have been other picks who ran and lost in the same year – notably Joe Biden who dropped out of the 2008 race right after the Iowa caucus – but Biden wasn't around long enough to harm Obama's race.
There are some advantages to selecting Sanders. Historically, the vice presidential pick was used to unite the party's differing factions. In this case, choosing Sanders would bulk up her apparently soft support from party activists. Sanders has also proven to be a strong motivator for young voters and an extremely effective fundraiser from new donors.
However, there is a basic and obvious reason to not choose a runner-up, as the campaign would have to explain away the criticisms and negative statements made by candidates against each other. In this case, because she was running an "oval office" type of campaign, Clinton was not particularly critical of Sanders. But the same can't be said for Sanders - there would be many embarrassing criticisms of Clinton that would have to be explained away. This isn't usually a personal issue - two candidates can probably get over any bad feelings for the good of their careers.
But personality is not what matters here. Republican TV ads will feature these critical comments in an attempt to damage her credibility. This tactic would play into the narrative that Donald Trump is already trying to craft around Clinton – that she chooses expediency and can't be trusted even with choosing a loyal running mate. Trump is already doing this, blasting Clinton for working with Obama despite her criticism of him during the 2008 campaign.
On the Republican side of the ledger, the issue would be magnified for Trump. More than any past candidate, Trump's criticisms of his primary opponents were vastly more negative and heavily personal attacks. But Trump's ability to defy the normal laws of politics in such matters may mean that he can get away with choosing someone like Chris Christie or Ted Cruz despite the withering criticism he laid down against them.
Under most circumstances, choosing a running mate whose own words can be used as an attack would be an unnecessary risk for the presidential contender, especially since studies have called into question whether vice presidential choices even have an impact on a presidential race. Richard Nixon, a man who ran for president or vice president five times, thought a vice presidential pick could only hurt the president.
In this case, Clinton is in a bit of pickle. While in the past, a candidate could safely ignore the complaints of the losing candidate's supporters, much as Barack Obama did when Clinton's supporters were upset in 2008, there seems to be a large group of Sanders' supporters who may not fall in line behind the party's candidate.
From the ancillary interviews, they could vote for a third party, they could vote for Trump or they could just stay home. Even with this reality, the "dream ticket" proposal may be too much of a challenge for Clinton. Sanders' comments may have been too negative, and as a socialist who has not really been tested in the primary race, he may have too much baggage for a general election. The result of this is that Clinton is likely to look for a candidate who she can hope will appeal to the economic argument of Sanders, but not take Sanders himself.
So far, there has been talk of Senators Elizabeth Warren and Sherrod Brown, both of whom can bolster her left-wing.
Dream tickets are always popular subject for campaign debates. But they are called dreams for a reason. They rarely happen in real life.