A growing number of House Democrats is taking President Obama to task for considering Social Security reform as part of bipartisan talks to raise the debt ceiling.

Sixty-two lower-chamber Democrats — roughly a third of the caucus — endorsed a letter to Obama Thursday urging his opposition to an extension in a cut to the payroll tax, which funds the popular seniors' benefit.

The lawmakers fear that such a cut “may be used as the first step in a larger battle to fundamentally dismantle Social Security.”

“We remain gravely concerned that yet another, unacceptable cut to Social Security’s revenue stream appears to be on the table,” the Democrats wrote. “As alternative measures would have the same net effect on deficits and the economy, there is simply no need to negotiate cuts to Social Security taxes.”

The letter was spearheaded by Reps. Lloyd Doggett (D-Texas), Ted Deutch (D-Fla.) and Mark Critz (D-Pa.), and attracted the support of a number of senior Democrats, including Reps. John Conyers Jr. (Mich.), Rosa DeLauro (Conn.), George Miller (Calif.), Barney Frank (Mass.) and Raúl Grijalva (D-Ariz.).

A similar letter penned in December was endorsed by fewer than 30 Democrats.

As Obama jousts with GOP leaders over legislation to slash deficits and raise the $14.3 trillion debt limit, the president has said repeatedly that all policy options remain on the table, including cuts in entitlement benefits.

Democratic aides on Thursday said Obama was moving closer to a deal that would raise the debt ceiling and cut trillions of dollars in federal spending, but would push tax reforms and revenue hikes to next year. Though the White House said such reports were incorrect, they added to Democratic ire.

Sen. Barbara MikulskiBarbara MikulskiClinton: White House slow-walking Russia sanctions Top Lobbyists 2017: Hired Guns Gore wishes Mikulski a happy birthday at 'Inconvenient Sequel' premiere MORE (D-Md.) summarized the sentiment, indicating many members were “volcanic” over the idea of a debt deal absent new revenues.

Democratic leaders from both chambers met again with Obama Thursday evening.

The current payroll-tax holiday was adopted by the White House in December as part of a deal with Senate Republicans to extend the George W. Bush-era tax cuts for all income levels. Under that agreement, workers' payroll taxes fell from 6.2 percent to 4.2 percent — a 32 percent reduction — this calendar year.

The cut does not apply to employers, who continue to pay the 6.2-percent rate.

The provision angered many Democrats, who said it sets a dangerous precedent because taxes are easier cut than restored. The concern was that anti-tax conservatives would lead the charge to extend the tax break beyond 2011, but Obama has championed that position instead.

The cut will reduce federal revenues by roughly $112 billion over the next two years, the Congressional Budget Office estimated — money the government will have to borrow to backfill the hole in the Social Security trust fund.

“Americans' contributions to Social Security should be used for Americans' Social Security benefits,” the Democrats wrote Thursday. “To do otherwise flies in the face of the time-tested wisdom of this program.”

—This post was updated at 7:39 a.m. on July 22.