One of the key stories of the day — perhaps the one that will have the greatest long term political impact — has to be that Mitt Romney has now fully embraced Paul Ryan’s Medicare plan.

After previously hedging on the Ryan plan, Romney is now fully declaring his support for it, as a way to wound the surging Newt Gingrich among conservative voters. Newt, you’ll recall, famously referred to the Ryan plan as “right wing social engineering,” and Romney, in a post on his Web site, has revived this Newt quote, and is suggesting he’d sign it into law as president, in order to portray himself as the only true conservative in the race.

“With friends like Newt, who needs the left?” the Romney Web site now blares.

The reason this matters: It will give Dems a weapon in the general election against Romney. “In order to make this attack, Mitt Romney has now given himself ownership of the Ryan plan,” Jed Lewison writes. “Let me say that again: Mitt Romney is now one hundred percent committed to Paul Ryan’s proposal to end Medicare and replace it with vouchers.”

Steve Benen added: “This is the line Democrats have waited eight months for Romney to take.”

Paul Begala summed up the thinking among Dems today. “The fact that Mitt Romney would call for essentially ending Medicare should disqualify him from the presidency in the eyes of millions of middle-class voters,” Begala told Talking Points Memo, adding that the Ryan plan is “the most toxic, anti-senior, anti-middle-class proposal I have seen from a major political party in years.”

A Dem operative emailed a one word response: “Rejoicing.”

One other question: Would having a candidate at the top of the ticket who fully embraces the Ryan plan be a boon to Dems in down ballot races? After all, when the House voted on the Ryan plan last spring, Republican operatives privately fretted that the vote would make House GOP incuments politically vulnerable. It’s true that since then there have been signs that House Republicans put this behind them. But now Romney has injected the Ryan proposal back into the public debate — and in the process has reminded everyone that the other top contender for the GOP nomination thinks the plan amounts to “right wing social engineering.”

Of course, there’s always the possibility that Romney will simply try to flip flop his way out of his support for the Ryan plan later, or fudge it somehow. But that would only give Dems more fodder to paint Romney as an ideological opportunist, and would likely enrage the GOP base. So Romney now owns the Ryan plan.

UPDATE: Romney’s surrogates are also attacking Newt over this. On MSNBC today, Governor John Sununu, a key Romney supporter, accused Gingrich of throwing Ryan “under the bus.”