UPDATE IV: Sen. Arlen Specter (D-PA) is the latest senator to sign on to the public option reconciliation letter, his spokesperson told Raw Story on Friday. The total number of signatories is now 19.

The move comes on the same day Senate Majority Leader Harry Reid (D-NV) announced in an important breakthrough that he's willing to use the procedure to pass the provision.

UPDATE III: Sen. Chuck Schumer (D-NY) has become the first member of the Democratic leadership to sign the public option letter, indicating its growing momentum.

In an email to supporters, reproduced by The Plum Line's Greg Sargent, Schumer said victory on the provision is "far from a done deal, but it’s an opportunity to break through the obstructionism Republicans have pushed for the past year."

UPDATE II: Sixteen senators have now signed the letter. The latest additions are Sens. Frank Lautenberg (D-NJ), Patrick Leahy (D-VT) and Barbara Mikulski (D-MD).

UPDATE: By Thursday morning there were 13 senators officially calling for a reconciliation vote on the public option, and two additional Democrats that have expressed support for the idea.

The new signatories are Sens. Dianne Feinstein (D-CA), Barbara Boxer (D-CA) and Jack Reed (D-RI). Sens. Amy Klobuchar (D-MN) and Ben Cardin (D-MD) have also indicated their support.

Feinstein became the 11th senator to officially sign the letter on Wednesday night. In explaining her decision, Feinstein pointed to the insurer Blue Cross's projected 39 percent rate hike for Californians.

"I can think of no better example of why we need health insurance reform," she said, according to the Huffington Post. "And this kind of behavior is a stark reminder of why any reform plan should establish a rate authority to keep insurance rates affordable."

Boxer and Reed followed, bringing the total to 13. The other two have announced they'll back reconciliation to pass a public option, but left unclear whether they would sign the letter.

Klobuchar said she supports "using reconciliation to pass the health reform bill with changes," in a statement to the Minnesota Independent. "I support the House bill version of the public option which is based on negotiated rates," she said.

But she hedged her bets and added that she's "also supportive of the President’s efforts to forge a bipartisan agreement."

Cardin also appears on board. "Senator Cardin has always been for a strong public option," said Cardin's spokeswoman Susan Sullam, Huffington Post reports. He also has long thought reconciliation was a viable option for passing strong health care reform."

This brings the total to 13 Democrats who have backed reconciliation to pass the hotly-debated public option, which was pronounced dead in December.

Raw Story's original article unfolds.

WASHINGTON -- As of Wednesday early afternoon, nine senators had signed a letter urging passage of the public health insurance option through reconciliation.

Make it ten.

Sen. Bernie Sanders's (I-VT) spokesman Michael Briggs confirmed to Raw Story that he will join Democratic Sens. Michael Bennett (CO), Kirsten Gillibrand (NY), Jeff Merkley (OR), Sherrod Brown (OH), Al Franken (MN), Pat Leahy, John Kerry (MA), Sheldon Whitehouse (RI) and Roland Burris (IL) in a last-ditch push for the provision.

The first four co-sponsored the letter and released it Tuesday, and The Plum Line's Greg Sargent confirmed Wednesday morning that the latter four will sign, just before Burris joined the list.

"The American people have made it clear that they want the option to buy their insurance through a Medicare-type, government-run public insurance plan," Sanders explained in a statement e-mailed to Raw Story.

"The public option is not just important for the individual consumer, it is an important mechanism to provide competition for the private, for-profit health insurance industry, to keep them honest, and to lower the overall cost of health care in our country," he said.

The signatories urge Senate Majority Leader Harry Reid (NV) to "bring for a vote before the full Senate a public health insurance option under budget reconciliation rules."

The public option, which has been the topic of explosive controversy throughout the health care deliberations, was passed in the House legislation but eliminated from the Senate version that was later approved.

The whole process hit a gridlock in January after the election of Republican Scott Brown to the senate, which gave the party the votes it needed to filibuster the final motion.

The letter continues, "There are four fundamental reasons why we support this approach – its potential for billions of dollars in cost savings; the growing need to increase competition and lower costs for the consumer; the history of using reconciliation for significant pieces of health care legislation; and the continued public support for a public option."

With Republicans poised to block another senate motion, Democrats have discussed using reconciliation -- which would require a simple majority of 51 senators -- to amend the bill before the House holds a final vote. A December poll found that six in ten Americans support the provision.

The liberal advocacy groups Progressive Change Campaign Committee and Democracy For America have endorsed the letter, which also encourages citizens to sign in the support of the idea. One hundred and nineteen members of Congress have also signed it.

Next Thursday President Obama will convene a bipartisan summit with Republican leaders to discuss the two parties' differences on the legislation. After initially equivocating, Republicans have confirmed they will attend.