Reflecting widespread and growing public support for the program, expanding Social Security benefits is becoming the Democratic Party line—particularly among Democratic Senate candidates hoping to gain an edge on Republicans in tight races this November.

The Democratic Party platform is also the strongest it has been in years in its support for the program.

"A few years ago, expanding Social Security was considered a radical idea. Now, it's the official position of the Democratic Party," commented the progressive group Social Security Works.

"Social Security is a sacred trust with our seniors that has lifted millions out of poverty, and they deserve to know that Social Security will be there for them when they need it," said Sen. Michael Bennet (D-Colo.) earlier this month, according to the Progressive Change Campaign Committee (PCCC). "With our seniors living longer, expanding Social Security and securing its long-term solvency will ensure that our most vulnerable Americans who have paid into Social Security all their lives are guaranteed the retirement they deserve."

Back in 2010, the Colorado senator dismissed Social Security as an "entitlement," telling NPR that "we can't deal with our budget crisis without dealing with our entitlements." Bennet is currently seeking reelection.

There has indeed been a sea change within the party, noted PCCC. "43 of 46 Senate Democrats favor expanding Social Security benefits," the organization observed in a press release earlier this month. "Nearly the entire Democratic Caucus voted on an amendment offered by Sens. Elizabeth Warren (D-Mass.) and Joe Manchin (D-W.V.) calling for it, and 120 House Democrats signed on to Rep. Jan Schakowsky's resolution in support of protecting and expanding Social Security."

Democrats running for Senate on promises to expand Social Security include Russ Feingold (D-Wis.), Ann Kirkpatrick (D-Ariz.), Tammy Duckworth (D-Ill.), Ted Strickland (D-Ohio), Kamala Harris (D-Calif.), Deborah Ross (D-N.C.), Patty Judge (D-Iowa), Katie McGinty (D-Pa.), Rep. Patrick Murphy (D-Fla.) and Sen. Patty Murray (D-Wash.), as well as Senator Bennet.

Many of those candidates are expressing support for expanding Social Security for the first time on record, PCCC observes.

Moreover, "[e]ven if Democrats' evolution on Social Security fails to win them control of the Senate, it is all but certain to achieve another progressive priority: ruling out a bipartisan budget deal to cut the program in the next Congress," Huffington Post notes.

The leftward shift is also taking place on the Hill. Democratic Representatives Linda T. Sánchez, Mark Pocan, and Michael M. Honda introduced legislation Friday that would expand Social Security benefits, Roll Call reports.

Sánchez said at a news conference that expanding benefits could offer an alternative to raising the retirement age.

"We think this is a common sense way to extend the solvency of Social Security and to increase benefits," said the California representative.

Progressives such as Sen. Bernie Sanders (I-Vt.) and Senator Warren have long pushed for expanding Social Security benefits, and President Barack Obama recently changed his mind on the topic, calling for an expansion of the program this past June.

Obama isn't the only centrist Democrat to shift course on Social Security, PCCC observed: "Unlike in 2014, the 2016 Democratic ticket is united around big, bold, progressive ideas like expanding Social Security benefits that will motivate voters this November," said PCCC co-founder Stephanie Taylor. "Years of progressive activism led to this moment."

Progressive voices are also calling on Democratic presidential candidate Hillary Clinton to join the growing chorus of politicians pushing for a dramatic expansion of social security—arguing that such a move would provide a much-needed boost to her campaign.