Democrats are bracing for a new assault from Republicans on the safety net that could come as early as this month — in particular, on disability insurance. In response, Senator Sherrod Brown is urging Dems to go on offense, by calling for an expansion of Social Security, rather than getting into an argument over how much to cut it.

Senator Orrin Hatch has requested a Finance Committee hearing into Social Social Security Disability Insurance — whose trust fund is set to be depleted soon — and Dems on the committee have agreed. It may take place this month, before the August recess.

Tomorrow, Brown will give a speech to the Center for American Progress at which he will attempt to preview the Republican criticism and outline a Dem response. Brown will argue that Republican criticism of SSDI is part of a “divide and conquer” strategy designed to pit supposedly undeserving recipients of disability insurance (who are allegedly defrauding the program, threatening its financial foundations) against deserving recipients of retirement benefits (i.e., the elderly). Brown will argue that this is part of a broader GOP assault on the basic principles undergirding social insurance programs, and urge Dems to counter with an expansive moral defense of them.

At the same time, CAP will also release a report tomorrow designed to push back on some Republican arguments about the program.

Republicans such as Senator Tom Coburn have argued that the disability program is being “gamed” by “scalawags,” and that a third of claims are bogus, putting the program in financial trouble. Republicans may use the coming hearing to argue for benefits cuts.

The report being issued by CAP will make several key arguments. The first is that by the standards of other countries, the U.S. system of disability insurance has very strict eligibility requirements. The second is that there are many broad causes for the program’s financial problems, including demographic trends (Baby Boomers “aging into their high disability years”) and labor-market changes (“the increase in women’s labor-force participation). The third is that there are a number of simple fixes that would put the program on sounder footing, a case that has been echoed by those who argue that a simple reallocation of payroll taxes has been repeatedly used to solve this problem before, and should be used again.

Beyond this, though, Senator Brown will argue that Dems should use the occasion to call for an expansion in retirement benefits. This would be long the lines of a proposal that would change the cap, increasing the amount paid in by the wealthy, to increase benefits by those in the lower and middle income groups who have seen retirement security eroded by the disappearance of savings, weakening of pensions, and other trends. Other Senate Democrats have offered another similar proposal.

As I reported recently, Dems are increasingly inclined to believe this could play well among a key demographic in midterm elections, i.e., older voters:

Two Democrats in tough Senate races — Mark Begich and Jeff Merkley – have already staked out aggressive postures on expanding Social Security. It’s also supported by Elizabeth Warren and Tom Harkin, and 70 members of the Congressional Progressive Caucus….the push to expand Social Security is emerging as another key issue — along with Wall Street accountability, stagnating wages, and the general sense that the gains of the recovery are going to the top — in the broader debate over whether the Democratic Party needs to move in a genuinely populist direction.

This idea was once mainly the province of a few liberal bloggers such as Atrios. The fact that it’s now getting play at a big event at the Center for American Progress — which will be livestreamed here — is another sign of how much the politics have shifted on the issue. Whether many leading Democrats will rally around the idea of expanding, rather than cutting, a popular social insurance program that has helped define the Democratic Party for decades is another question entirely.

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UPDATE: Here some excerpts from Senator Brown’s scheduled speech, provided by his office:

There is a quiet, covert war being waged on Social Security. The tactic? Divide and conquer.

They have made so-called “structural reforms” their goal. But it’s up to us to call it like it is: privatization.

When detractors of Social Security sought to privatize the program in 2005, progressives organized and fought back. When they used the budget as an excuse to attempt to implement cuts like the Chained-CPI, we organized and fought back.

Today, detractors try to use Social Security Disability Insurance as a back door to cut the program as a whole. And, we will organize and fight back against today’s attacks aimed at some of the most vulnerable Americans: the disabled.

Opponents of Social Security realize that when they attack the program head on, they lose. So, their strategy is two-fold: First, convince the public that the disability insurance program is bankrupt. Second, separate Social Security from disability insurance in the eyes of voters.

We need to recognize these attacks for what they are – backdoor attempts to weaken Social Security by dismantling disability insurance.

Reallocation is not controversial, but detractors will do anything to manufacture a crisis out of a routine administrative function.

The average disabled worker receives less than $300 per week. One in five lives in poverty, and in fact, nearly half of disabled workers who are younger than 50 are poor or near poor.

That’s why it is our responsibility to make sure that the programs we support are efficient and effective, so where fraud exists, we need to address it. If we don’t address the lies, this will have serious repercussions that harm the most vulnerable Americans.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.