The idea has long remained outside the boundaries of acceptable Beltway discourse, but more and more Dems are coming around to it: Instead of getting drawn onto GOP austerity turf — into a debate over how much to cut Social Security — Dems should instead go on offense on the issue and push for an expansion of the program.

Today, I’m told, Senate Democrats will introduce a proposal to expand Social Security benefits for certain groups — and it is picking up the support of a member of the Democratic leadership, Senator Patty Murray of Washington State. Senator Mark Begich of Alaska will take the lead on the proposal, and he and Murry will speak about it on the Senate floor this afternoon.

The new proposal is called the Retirement and Income Security Enhancements Act, or RAISE Act, and it would increase benefits specifically for groups who have seen their retirement security eroded by recent economic trends such as the transition to two-earner families, stagnating wages, declining savings, and the erosion of pensions. It would increase benefits for many divorced spouses, and widows and widowers, and would extend benefit eligibility for some children of retired, disabled or deceased workers –  to be paid for by a two-percent payroll tax on earnings over $400,000, which is also designed to help shore up the program’s long-term finances.

This afternoon on the Senate floor, Murray will say:

“Wages have stayed flat — or even declined for some. And fewer companies offer the kinds of generous pension plans that used to help so many workers say financially secure…For 75 years our Social Security system has offered millions of seniors and their families a foundation of financial security.[...]

“But a lot has changed in those 75 years….today most families have two earners. Because Social Security was designed for single earner families, surviving spouses in families where both adults worked may receive less in benefits than they deserve…The RAISE Act would make some commonsense updates to ensure our Social Security system is doing everything possible to help seniors and their families.”

The framing reflects a growing sense among Dems that they can win the argument over entitlements by framing it as a battle over which party would boost the retirement security of the elderly, and which would erode it. Republicans will surely accuse Dems of fiscal recklessness, but they will counter that Republicans are merely displaying their priorities once again by opposing higher taxes on the wealthy to bolster the economic security of those hurt by the Great Recession.

To be sure, it’s anybody’s guess whether this proposal will ever get a Senate vote. Still, today’s measure represents another step forward for an idea that originally was only pushed by liberal bloggers such as Atrios. It’s also telling that Begich, in particular, is pushing the proposal, because he is locked in one of the toughest reelection fights in the country. Begich recently said he actually views expanding Social Security as good politics for Dems, arguing: “Are we for or against helping seniors have a dignified life in their later years? I’m for that.”

It’s true that many Democrats remain reluctant to embrace the proposal. But an increasing number of Dems — including Elizabeth Warren, Sherrod Brown, Tom Harkin, Jeff Merkley and 70 members of the Congressional Progressive Caucus — believe it’s good politics to push for an expansion of a hugely popular program that has been central to the Democratic Party’s identity for many decades. Indeed, whether or not the proposal goes anywhere, the push to expand Social Security is emerging as another key issue in the broader debate among Dems and liberals over whether the Democratic Party needs to get serious about moving in a genuinely populist direction, which could resonate into 2016 and beyond.

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* SOME DEMS SEE POSSIBLE UPSIDE IN EPA RULES: Carl Hulse weighs in with a must-read reporting that in certain Senate races, such as Colorado, Iowa, and Michigan, Democrats actually see possible political benefits in embracing the need to save the planet:

Democratic strategists say that climate change issues — and Mr. Obama’s decision to push the pollution debate to center stage — will have even greater benefits for Democrats in presidential campaigns and future congressional elections. In interviews, they said the climate issue has become a way for voters to judge a candidate’s character and broader outlook on the world. Officials compared the issue to the growing acceptance of same-sex marriage, especially among young people and suburban women.

Multiple GOP Senate candidates are now acknowledging the existence of climate change but fudging on its cause. Meanwhile, there are reasons to believe that the debate over EPA rules will hurt Republicans more in 2016 than it will hurt Republicans in 2014.

* POLL: IMMIGRATION REFORM NOT LIABILITY AMONG GOP VOTERS: A new Public Religion Research Institute poll finds 62 percent of Americans support a path to citizenship for illegal immigrants in the U.S. Even 51 percent of Republicans support it. And:

Even among Republican voters, opposing immigration reform carries more political risk than benefit. Nearly half (46%) of Republican voters say they would be less likely to vote for a candidate who opposes immigration reform that includes a path to citizenship, while 21% say they would be more likely to support such a candidate. Three-in-ten (30%) Republican voters say it would not make a difference to their vote either way.

Yet we are told again and again by that Republicans can’t possibly risk tackling reform now, because it would supposedly trigger “civil war” in the party and imperil the GOP’s inevitable Obamacare-fueled victory this fall.

* PRO-REFORM GROUPS RATCHET UP PRESSURE ON GOP: The Wall Street Journal reports that high-tech groups are going up with two new ads on national cable calling for immigration reform. This one frames the choice as one between deporting 11 million people and restoring sanity to our immigration system for the good of the economy:

The ad, however, targets “Congress,” rather than “Republicans,” who will respond to the ad with a shrug.

* BERGDAHL’S HOME TOWN PLEADS FOR PATIENCE: USA Today has a good piece recounting that people from Bowe Bergdahl’s Idaho town would just like the nation to reserve judgment until the facts are in, and are tired of getting pummeled by outside criticism, which has included threatening emails and calls. “We’re just a small community that cares about those who go out and fight for us,” the head of the Chamber of Commerce says. “They’re calling our town a traitor town.”

* HAGAN GOES UP WITH NEW AD: Embattled Senator Kay Hagan is up with a new ad featuring a retired Marine talking about Hagan’s work in securing a law, named after his dead daughter, that helped Marines and family members sickened by contaminated water at Camp Lejeune. Hagan has cut a far lower profile than embattled incumbents such as Mary Landrieu and Mark Pryor, so she really has her work cut out for her in reestablishing a bond with voters that might insulate her from the bad national environment.

* AND AMERICANS OPPOSE BERGDAHL SWAP: A new CBS News poll finds that 45 percent of Americans disapprove of the deal, while 37 percent approve of it. Fifty-six percent saying the U.S. paid too high a price to get Bergdahl back, while 72 percent say Obama should have notified Congress.

This sort of finding could lead Congressional Democrats to continue to criticize the deal, which we may hear more of today after Senate Dems on the Armed Services Committee get their classified briefing from military officials this morning.

What else?

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