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CNN  — 

On Thursday afternoon, barraged by reporters asking about criticism from the  left for his opposition to a $3.5 trillion budget package, West Virginia Democratic Sen. Joe Manchin offered progressives some advice:

If they want a bigger, more costly bill, they should “elect more liberals.”

Which is a good line! But Manchin misses the mark when it comes to the modern Senate, which has grown far more partisan and watched its moderate center erode away.

While the polarization has been asymmetric – Republicans in the Senate (and even more so in the House) have grown more conservative than Democrats have grown liberal – the results are the same: The ideological middle is no more.

According to GovTrack’s ideology ratings, there are only two sitting Democratic senators – Manchin and Arizona Sen. Kyrsten Sinema – who rank more conservatively than the least conservative Republican. The middle is slightly more robust on the Republican side, with six GOP senators ranking more liberally than the least liberal Democrat.

That lack of an ideological center is borne out in other ways as well.

This chart from VoteView paints the reality of the disappearing middle in stark terms, with every senator ranked based on their voting record in the current Congress.

The number of states that send a split partisan Senate delegation to Washington is just six, the lowest it has been in more than 100 years, according to a Pew Research Center analysis. 

(Sidebar: These Senate numbers are consistent with the overall march of people to their respective partisan camps. A Pew survey earlier this decade showed that 94% of Democrats were more liberal than the median Republican while 92% of Republicans were more conservative than the average Democrat.)

Those numbers too much for you? Just go back two decades and look at the senators serving. (Thanks to VoteView, we can do this easily.)

In the 107th Congress, which was in session from 2001 to 2003, Georgia’s Zell Miller, a Democrat, was far more conservative than at least a half dozen Republicans. The ideology of the voting records of Democratic Nebraska Sen. Ben Nelson was virtually indistinguishable from those of Republican Sens. Jim Jeffords of Vermont and Lincoln Chafee of Rhode Island.

There were simply a whole lot more moderates – in both parties – back then.

The issue, then, with the Senate is not that there aren’t enough liberals. Or conservatives. By the numbers, there are far more than there were a decade and especially two decades ago.

The issue is that there are so few moderates – especially on the Democratic side – that when the margin between the parties is narrow (as it is now), a single senator, like Manchin, has almost total power. 

The Point: The disappearing middle in the Senate has consequences. And one of the big ones is to make the few senators – like Manchin or Sinema – who still peg themselves as centrists hugely powerful in moments just like this.