The Republican presidential candidates are about to debate again, which is always entertaining for Democrats to watch (in horrified fashion, of course). The current state of the Republican race is that it has reached the point where loud voices are beginning to demand that both Marco Rubio and (to a lesser extent, at least so far) John Kasich bow out of the race. The logic is persuasive, to all but the candidates' inner circles: "You can't win the nomination, and your continued presence in the race is preventing our best shot at dethroning Donald Trump, so for the good of the party you should drop out." Look for this to be the one thing Trump and Ted Cruz agree on, tonight -- that Rubio and Kasich should just pack it up and go home. But while this argument is being made on the Republican side, for Democrats the argument just isn't going to be valid. Because the one thing Hillary Clinton cannot do is to call on Bernie Sanders to exit the race.

There are two reasons why I say this, one technical and one personal (to Clinton, not to me). The technical reason why Hillary just doesn't have the option to ask Sanders to end his run is that he's the only opponent she has left. On the Republican side, they have winnowed their numbers from a whopping 17 candidates down to only four. Of these four, one is dominating. The argument is to pare the contest down to a true head-to-head matchup, which might actually give Cruz a shot at taking down Trump. This argument does not work on the Democratic side. Indeed, if Martin O'Malley or Jim Webb had stayed in the race this long, you might now be hearing Bernie making the argument that they should drop out and give him a shot at beating Clinton.

Which brings us to our second reason. Bernie Sanders certainly could have made the argument to Webb or O'Malley to drop out, because it's a valid argument for him to make and he does not face any charge of hypocrisy in making it. This is simply not so with Hillary Clinton. Even if there were more people still in the race (negating the first technical reason why she can't make this argument, in other words), Hillary still can not make this argument with a straight face.

The reason is simple, and can be easily expressed as: "2008." Hillary Clinton has a history on the presidential campaign trail. Her own history precludes her from now arguing that Sanders should just step aside and let her focus all her energy on the Republicans. She cannot make this argument because she defied this argument when she ran against Barack Obama.

Hillary Clinton refused to give up, in 2008. Whether you see this as a good thing or a bad thing, it happened. Clinton campaigned until the very end, even when many within her party were strongly arguing that it was mathematically impossible for her to catch up to Barack Obama in the delegate count. She campaigned until the last primary in June, and then she waited for weeks before she finally came out and publicly endorsed Obama for president. No matter what light you see this history in, that's the way it played out.

Which is why it would be the absolute height of hypocrisy for her to now make the argument against Bernie Sanders staying in until the exact same last minute as she did. Why, after all, should she now get a free ride when she did not do the same for Barack Obama when she had the chance?

Others can make the argument for Bernie to withdraw -- and they likely will, starting next Wednesday morning, after another big round of primaries happens. But no Hillary Clinton campaign official should even hint at this argument, and the candidate herself would rightly be ridiculed if she tries to advance the idea.

Although fervent Obama supporters seethed (back in 2008) at how Hillary stayed in the race far longer than they thought she should, it undeniably made him a better candidate. It improved his performance, which likely helped him by the time he did face John McCain. Hillary pulled no punches in attacking Obama (something most have forgotten by now), and his response was initially fairly weak, but got better over time. So Clinton's long campaign, in the end, benefited the Democratic Party as a whole.

To give proper credit where it is deserved, Clinton did a lot to mend the rifts within the Democratic Party after she endorsed Obama. The most touching moment in the entire campaign (for me, at least) was when Clinton surprised everyone on the convention floor. Traditionally, as the delegates' votes are announced from each successive state, the honor of being the group of delegates which puts the candidate over the top (giving him the majority he needs to secure the nomination) is given to the candidate's home state. But in a well-choreographed surprise, Illinois deferred this honor to the state of New York. The New York delegate leader stood up and personally deferred the announcement of the vote totals -- to the superdelegate senator from New York, Hillary Clinton. Clinton herself announced the votes that clinched the nomination for Barack Obama. As I said, it was a moving moment which did a lot to convince Clinton's supporters to wholeheartedly support Obama in the general election (remember, these were the supposed days of the "Party Unity, My Ass" PUMA Democrats). This all might serve as an abject lesson for Bernie supporters this year, if Clinton does secure the nomination.

But Bernie can stay in the race as long as he likes, before we get to that point. Even if Clinton continues to rack up hundreds more delegates than Bernie, his decision to keep fighting cannot be influenced by Clinton herself.

How much the presence of Sanders in the race for a long time helps or hurts Clinton is still a subject open for discussion. Is he "pulling her to the left" too much? Or is he forcing her to adopt positions that are actually very mainstream, and will help her in November? One thing is clear to me, Sanders (2016) is no Clinton (2008). By this, I mean Hillary was a much fiercer opponent to Barack Obama than Bernie is to Hillary now. Bernie is not comfortable with negative campaigning, and has continually tried to raise the level of the debate above personalities. He has also famously refused to fully exploit Clinton's email weaknesses. Neither of these things will be true when Clinton has to take on Trump or Cruz. They will hit her with everything they can think of. So no matter how long Bernie stays in, it's not going to be anything like what she'll face in the general election. Clinton, on the other hand, toughened Obama up to a remarkable degree. In fact, by the time the general election came around, pretty much all the attacks on Obama fell flat precisely because they had already been tried by Clinton. They were "old news" in the political debate, and therefore they didn't sting as much as they could have. Clinton now isn't getting the same preparation from Sanders that she put Obama through.

Still, Sanders staying in the race for a long time will probably benefit Clinton. Even if that weren't true, however, the Clinton campaign simply cannot -- at any time for the rest of the primary season -- call on Bernie to end his run. Not only would it be incredibly hypocritical of Clinton to suggest, it would also annoy the heck out of Bernie's fervent base. And if Clinton is going to win in November, she's really going to need their support. Calling for Sanders to step down would be incredibly disrespectful and insulting, to put it mildly. She blazed the path of staying in until the very end, and now she's going to have to walk that path herself. We'll see Republicans calling on Republicans to drop out in tonight's debate, but we won't see this tactic from Hillary Clinton -- because calling on Bernie to clear the nomination path for her is the one thing which her campaign simply cannot do.

-- Chris Weigant

Follow Chris on Twitter: @ChrisWeigant