A coalition of progressive politicians, policy experts and grass-roots advocates started a campaign three years ago facing very long odds. They proposed expanding Social Security retirement benefits for millions of Americans.

Mainstream thinking in Washington at the time ran in the opposite direction: Social Security benefits should be cut as part of a "grand bargain" to get the federal deficit under control. Nearly all Republicans supported this consensus view, as did many Democrats with moderate or conservative leanings. President Obama also bought in to this thinking, signaling that he was open to benefit cuts as part of a big budget deal.

Meanwhile, it has become more evident that retirement security is eroding for many Americans. The value of Social Security benefits has shrunk by roughly 25 percent because of benefit cuts put in place when the program was last reformed in 1983. At the same time, the share of households receiving guaranteed income from traditional pensions has plunged.

And a very large segment of the near-retirement community has negligible savings: Among workers age 55 or older, just 30 percent have saved more than $250,000, according to the Employee Benefit Research Institute.

But the tide has turned — big time — and it is starting to look as if Social Security expansion really could happen.

Dramatic shift

Obama has endorsed the idea, signifying that it has been pushed to the center of the Democratic agenda. Much of the credit for this belongs to Sen. Bernie Sanders, an early sponsor of expansion legislation.

Neither of the presumptive nominees for president favors cutting benefits — although with Donald Trump, it is difficult to tell. Hillary Clinton has slowly moved from "no cuts" to embracing expansion.

If Democrats win the White House on Nov. 8 and regain control of either chamber of Congress, they will be in position to propose expansion as part of a much-needed, broader Social Security reform package. That is a dramatic shift from the Social Security battles of the past decade, when progressives were forced to play defense against proposals to reduce annual cost-of-living adjustments, increase retirement ages — and turn the program into a system of private savings accounts.

What would expansion look like? Many advocates want to see an across-the-board boost, with progressivity built in to avoid large increases to the wealthiest households.

Sanders' bill would boost benefits across the board by roughly $65 per month and adopt a more generous annual COLA that accounts for the higher health care costs faced by seniors. It also would provide a minimum benefit aimed at reducing senior poverty rates. Clinton proposes expanded benefits targeting widows to address the greater longevity risk facing women. She also favors awarding Social Security work credits for women who take time from jobs to care for family.

A winning idea

Congress will have to address Social Security sometime soon. The program's two key trust funds — for retirement and disability programs — are on track to be exhausted in 2034. Without reforms, the program would have only enough continuing tax revenue to pay 79 percent of promised benefits.

The smart approach is to address the shortfall and expansion simultaneously. And paying for all this is not difficult. For example, the Sanders bill extends the trust funds' solvency for 50 years by eliminating the cap on payroll taxes. Currently, no income over $118,500 is taxed. Sanders would leave that cap where it is but subject income above $250,000 to the tax.

Other revenue ideas that have been floated include a very gradual increase in payroll tax rates, expansion of estate taxes and investing a small portion of the trust fund in stocks to boost returns (currently, reserves can be invested only in low-return Treasury notes).

The key question right now: How big an issue will Democrats make of Social Security in the fall election? Expansion is a winning issue with voters — strong majorities of Americans favor the idea, and most would be willing to pay higher taxes to finance it, according to polling by the National Academy of Social Insurance. That is true of both Democratic and Republican voters.

The Democrats should build a strong Social Security expansion plank into their platform and then campaign hard on it through autumn. Then they would be in a position to propose legislation through an open process — not the backdoor "grand bargain" talks that politicians have used in the past to cover their tracks on cutbacks.

That could force expansion opponents to explain to their older voters why they oppose putting $70 extra in the pockets of grandmothers every month — or why they are OK with allowing Social Security to keep veering toward a 21 percent benefit cut in 2034.

Any bets on how that fight turns out?

Mark Miller is a Reuters columnist.