This new ad from the National Republican Congressional Committee has to be the most striking attack on Dems from the left yet — it accuses Dem Rep. Lois Capps of voting for “the most extreme plan” on Medicare:

The ad says:

Congress is debating big changes for Medicare. And Congresswoman Lois Capps voted for the most extreme plan. Capps voted for the plan the media says would decimate Medicare. Independent experts say Capps’ Medicare plan will cut Medicare benefits by 17 percent immediately — on top of the $500 billion Capps cut from Medicare last year.

The “most extreme plan” Capps is accused of supporting appears to be a reference to the oft-repeated claim by Republicans that Dems support doing nothing on Medicare — a claim that Post fact checker Glenn Kessler recently debunked. The ad’s claim that the “media” says the Dem “plan” would “decimate Medicare” is a reference to a single editorial in Investors Business Daily that makes the more general claim that Dems “would decimate Medicare by default.”

All that aside, though, this ad attacking Dems from the left — which follows another GOP spot that accuses Dems of wanting to shred the “social safety net” — is basically an acknowledgment that Dems have won the argument over Medicare. Dems have successfully cast the battle over Medicare as one between those who would save the program and those who would destroy it — or at least transform it so fundamentally that it would cease to exist.

As this ad shows, the GOP’s response is to muddy the waters by claiming that both sides agree Medicare needs to be cut and that the only difference is over the details — indeed, the GOP is now claiming that the Dem plan is more extreme than the Ryan plan. Cutting Medicare is now “extreme”; Dems have already proven themselves willing to take that step; Republicans won’t let Dems get away with it.

The question now is whether Dems will take this sort of thing as a sign that they maintain an advantage on Medicare that would be badly undermined if they agree to any significant Medicare benefits cuts. If they do, it will only give Republicans more grist to keep attacking them from the left in exactly this fashion, and muddle the contrast between the parties on a popular entitlement program that has been a pillar of the Democratic Party’s identity for decades.

In other Ryan-related news, the League of Conservation Voters is up with a new spot in Wisconsin hitting Ryan for supporting billions in oil industry subsidies while his family profited from companies receiving those tax breaks.