After Hillary Clinton lost the Wisconsin primary in early April, backers of the former Secretary of State began publicly worrying about how the nomination process was unfolding. Their concern wasn't so much that Clinton could lose the nomination to Sen. Bernie Sanders (I-Vt.) but that she was at risk of forfeiting the incredible asset afforded to her by the chaos in the Republican primary: time.

David Plouffe, who managed Barack Obama’s 2008 campaign, said that by May, Clinton needed to be not just thinking about the general election but actively planning for it. "I think you’ve got to start having your state directors in place. You have got to start traveling to those [battleground] states. You’ve got to start communicating to voters in those states,” he said. “You can’t wait till June 8 to start the general election. … That would be a feat of epic political malpractice.”

Clinton's advisers agreed (and still do) with Plouffe's point. But any pivot to the general election is premised on things being cleanly resolved in the primary. While Clinton may want to start planning for Donald Trump -- or whoever emerges on the Republican ticket, Sanders has made it clear he's not ready to pave that tidy path.

In recent days, the senator has reiterated that he intends to see the race through to the convention. And instead of dulling the barbs between now and then, his camp seems to be sharpening them. On Tuesday, the campaign sent out a fundraising email that accused Clinton and her backers of painting Sanders' supporters as "traitors." But the coup de grace was including a picture of Clinton and Trump at the GOP front-runner's most recent wedding.

"Let me be clear, there is one candidate in this Democratic primary who Donald Trump said would make a "great president," and it’s not Bernie Sanders," the email reads. The photo caption reads: "Donald Trump and the one candidate in this race he said would make a "great president." (Spoiler alert: that candidate is Clinton.)

Bernie Sanders Bernie Sanders' latest fundraising email.

The picture is familiar to anyone who has followed the GOP primary. It's been used repeatedly against Trump to show that he isn't actually a Republican, since no self-respecting conservative would rub elbows with a boogeyman of the movement. In inverting that attack against Clinton, Sanders is doing virtually the same: suggesting she's veered far away from progressivism, since what self-respecting progressive would get cozy with Trump?!

This is an escalation of attacks, not merely a continuation of the ones that have painted Clinton as part of upper crust New York society. Those made Clinton out to be a corporate-minded Democrat, but still a Democrat. This makes the case that she’s Trump's Platonic ideal of a commander in chief.

The immediate reaction from Democrats was that Sanders was basically desperate, his campaign engaging in the last-ditch efforts of a losing enterprise.

The Sanders Campaign is very publicly going through all the stages of grief that come with losing a close campaign https://t.co/VAfUHtQxHk — Dan Pfeiffer (@danpfeiffer) April 26, 2016

Perhaps so. But for folks like Plouffe and many on the Clinton campaign, it signals some trouble too. Sanders isn't going to go quietly -- at least not before the convention gathers.

"Bernie is going to fight through the narrow path we have to the nomination because there is also only one candidate who believes health care should be a right for everyone in this country, that kids of all backgrounds should be able to go to college without crushing debt, and that we cannot transform a corrupt system by taking its money," reads the fundraising email.