So today five Senate Democrats will call on Vice President Joe Biden to make it crystal clear that Ryancare is not on the table in the deficit reduction talks he’s presiding over.

Senate Republicans have said in various ways that Paul Ryan’s plan, or elements of it, should be part of any final deal that Dems and Republicans agree to on deficit reduction. In response, Dem senators Claire McCaskill, Ben Cardin, Jon Tester, Bill Nelson and Sherrod Brown have sent a letter to Biden calling on him to reaffirm his commitment to keeping Medicare fundamentally intact.

I’ll bring you the letter when I get it, but for now, suffice it to say that this is an important development.

For one thing, it’s key to have moderate Dem senators drawing such a hard line. It signals that they have decided that the Ryan plan is so toxic that they can oppose it without worrying about negative consequences for their reelection campaigns. This “shows that there is no daylight to try and pick off moderate Dem support on this issue,” as Eddie Vale of ProtectYourCare puts it.

For another, this offers an opportunity for the White House to state clearly that transforming Medicare is, indeed, off the table in the Biden talks. Asked for comment, a White House official said that “the President has made his opposition to Ryan’s plan quite clear.”

To be sure, Dems aren’t necessarily doing this because they worry that the White House is open to caving on Ryancare during the deficit talks. Ever since his big deficit reduction speech, Obama has drawn a sharp contrast between the Ryan vision and the Dem one — suggesting the White House won’t agree to anything that risks compromising the sharpness of that contrast in a fundamental and meaningful way. But Dems would still like the White House to respond forcefully to the Senate GOP’s demands that Ryancare be part of the equation by ruling it out completely.

Of course, other Dems expect the White House to refrain from drawing any such lines publicly right now, because they are making nice with Republicans in the quest for a deal. Either way, a significant development. More when I learn it.