Hillary Clinton has been absolutely crushing Bernie Sanders when it comes to endorsements from elected Democrats. She has 34 of the US Senate's 44 Democrats, 10 of the 18 Democratic governors, and 120 of the US House's 188 Democrats.

But Clinton's most surprising endorsement is the 34th and most recent senator to get on board: Sherrod Brown of Ohio.

"From opposing unfair trade deals to fighting for a fair financial system, Hillary Clinton has shown she puts working families first," Brown said in a statement announcing the endorsement on Tuesday. "She knows as president that her first job will be creating jobs for the middle class. I am proud to endorse her today because I know she will keep Ohio moving forward."

Bernie Sanders is losing the invisible primary really, really badly

On one level, this isn't terribly surprising. While Brown didn't endorse Clinton (or anyone) in the 2008 primary, the race isn't nearly as close this time around, and as a solidly liberal senator from the swingiest of swing states, an endorsement sets him up well as a VP prospect.

But the move is also an indication of just how tough Sanders is finding it to get even ideologically sympathetic elected officials on his side.

Bernie and Sherrod Brown agree on almost literally everything. They're both vocally pro-union, anti-free trade populists. And they're personally close. They were in the House together for 14 years (where they were both members of the Progressive Caucus), and both got elected to the Senate in 2006, so they've had more than two decades' worth of opportunities to collaborate on just about every economic policy issue you can imagine.

"What’s very clear is that Sherrod Brown knows which side of the struggle he is on"

This very month, they introduced a bill together to make union organizing easier. Just a month ago, they introduced legislation together repealing Obamacare's "Cadillac tax," a move pushed by their supporters in labor unions whose members have to pay the tax; they also introduced an amendment trying to strip the tax when Obamacare when being debated in 2009. They jointly led the unsuccessful effort this past spring to kill fast-track authority for the Trans-Pacific Partnership trade deal.

"I’ve known him for many years," Sanders told Chris Hayes for a profile of Brown back in 2005. "What’s very clear is that Sherrod Brown knows which side of the struggle he is on."

If any senator were going to endorse Sanders, it would be Sherrod Brown. And yet here he is, endorsing Clinton.

It's not getting better anytime soon

This is a broader problem than just Brown, though. Other vocal congressional progressives have flocked to endorse Hillary:

Sen. Tammy Baldwin (D-WI), who was in the Congressional Progressive Caucus with Sanders as a House member

Rep. Jim McGovern (D-MA), a Progressive Caucus member and Congress's premier defender of food stamps

Rep. Jan Schakowsky (D-IL), a Progressive Caucus member and the prime congressional advocate of a public option

Sen. Barbara Boxer (D-CA), who sponsored carbon tax legislation with Sanders

Rep. José Serrano (D-NY), the vocally pro-Castro and pro-Chávez congressman from the Bronx who was one of only two House members in 2004 to endorse Al Sharpton for president

To a large degree, this is a testament to just how many connections Clinton maintains with different important factions in the Democratic Party. As far back as 2013, women senators were announcing their support for her bid. Her time as senator from New York means liberal politicians from New York City like Serrano feel obliged to offer her their endorsements. Mayor Bill de Blasio — who, along with Sanders and Elizabeth Warren, is one of the most prominent progressive Democrats in the country — is reportedly planning to endorse Clinton soon. That doesn't just reflect regional loyalties: He ran Clinton's 2000 Senate campaign.

By contrast, Sanders has been endorsed by a total of two major elected officials: Congressional Progressive Caucus co-chairs Reps. Keith Ellison (D-MN) and Raúl Grijalva (D-AZ). He couldn't even keep Vermont Gov. Peter Shumlin, a die-hard progressive who tried to implement single-payer health care in the state, from endorsing Clinton.

It's still theoretically possible that Sanders could nab an endorsement from Elizabeth Warren, the last big-deal progressive whose support is still up for grabs. But it's unclear why she would. She signed that 2013 letter from women Democratic senators expressing support for Clinton, and with her position in the Democratic leadership, it's doubtful she'd want to alienate herself from her colleagues by being literally the only senator to back Bernie.

Not everything is looking bleak for Sanders. He's still ahead in New Hampshire and not far behind Clinton in Iowa. But if you believe the political scientists who argue that party elites ultimately control the nomination process, then it follows that he has little hope of ultimately beating Clinton.