Rep. Chris Van Hollen, the ranking Democrat on the House Budget Committee, is running for the Senate seat currently occupied by Maryland Sen. Barbara Mikulski. He is also running from his record – as a supporter of the “Simpson-Bowles” plan to cut Social Security and top tax rates, a once-favored economic agenda among Washington insiders and some wealthy private interests.

That’s a smart move – but Rep. Van Hollen has more ground to cover.As we reported last week, progressive groups like the Progressive Change Campaign Committee and Democracy for America strongly encouraged Rep. Donna Edwards to enter the Senate race. She did – with a video announcement that directly challenged Van Hollen’s support of recommendations named for the leaders of the 2010 White House deficit commission, Republican ex-senator Alan Simpson and Democratic operative Erskine Bowles.

Van Hollen had said he was “willing to consider” entitlement cuts. Edwards’ video response was blunt: She boasted that she was known for “standing up to anyone who would compromise away Social Security and Medicare – no ifs, ands, buts, or ‘willing to considers.’”

Van Hollen apparently got the message. This week he endorsed legislation from Rep. John Larson (D-Conn.) that would expand Social Security benefits by removing the payroll tax cap (which currently exempts income above $118,500 per year) for earnings of $400,000 and above.

“You don’t save Social Security by cutting Social Security,” said Van Hollen, in a striking reversal from his past positions.

He is to be commended for this shift. It’s the right policy, and polling shows overwhelming support for a bill like Rep. Larson’s. A survey conducted last year by Lake Research Partners shows that three-quarters of Republicans and independents, as well as 90 percent of Democrats, support this approach. And, as Celinda Lake told me in an interview last September, Social Security is a “valence issue” that changes votes.

Social Security is clearly a major issue in the Maryland race. What are the national implications?

1. The debate has shifted.
In the insular world of the Beltway, the Simpson-Bowles plan was totemic in the early years of this decade. Support for it was a sign of “seriousness” among those who supported Social Security cuts and its other ideas. Simpson-Bowles’ neoliberal policy prescriptions were popular with the wealthy backers of its self-described “centrist” agenda, as well as with the arbiters of Washington’s self-limiting political debate.Social Security expansion, on the other hand, was considered a “fringe” idea. Even when polls were commissioned showing its popularity, and even after economists demonstrated it was feasible and wise, there were deep-seated resistance to the idea among influential people in politics and the media.

That’s changed. Social Security cuts no longer hold “bipartisan” appeal – and a growing number of proposals and bills now call for Social Security expansion.

2. The “Overton window” has moved.
The change in the Social Security debate reflects a broader shift. The range of political thought included in the mainstream political debate – for the public, and more recently for political and media insiders – is sometimes described as the “Overton window.” It shifted sharply rightward after the 1980s, as Republicans embraced a more extreme conservative platform and corporate-funded Democrats gained more influence over their party.

The public has been moving the other way, however, especially since the 2008 financial crisis. Voters are increasingly embracing an economically populist agenda, according to a number of polls, and a growing number of politicians are following suit. This shift can be seen in the debate over Social Security, but it is also reflected in public opinion on issues that range from taxation to trade.

3. The public has a long memory – and needs commitment from its candidates.
Rep. Van Hollen’s public advocacy of Simpson-Bowles reached its height in 2012, but he’s being forced to defend himself for it three years later. Three years is a long time in political cycles.When the political mood shifts, the public doesn’t necessarily forget. That’s why it was wise for Rep. Van Hollen to support Social Security expansion. But it’s also why he needs to do more: The public has grown used to seeing politicians express support for a program at election time, only to offer it up for sacrifice later.

Sure, politicians must sometimes compromise. But the public needs to know that some programs are off-limits – especially today, with extremist Republicans riding herd over their party. That’s why groups like MoveOn.org and CREDO have called upon Rep. Van Hollen to state unequivocally that he will not support Social Security cuts in the future. Better to have no budget deal at all than to have one that extracts a lifetime’s worth of sacrifice from America’s seniors.

As CREDO political director Becky Bond said recently, “Van Hollen needs to draw a line in the sand and make it clear that he will vote against any bill that cuts Social Security or Medicare benefits.”

4. Progressive politicians are changing the debate.
It is unlikely that Rep. Van Hollen would have shifted his position on Social Security if Rep. Edwards had not challenged him. Edwards represents the American tradition of independently minded progressive leaders – leaders like Massachusetts Sen. Elizabeth Warren, Reps. Keith Ellison (Minn.) and Raul Grijalva (Ariz.) of the Congressional Progressive Caucus, Sen. Bernie Sanders, Florida Rep. Alan Grayson, Ohio Sen. Sherrod Brown, and a number of other courageous officials.

The political discourse won’t change unless party hierarchies and artificially imposed “consensus thinking” is challenged by leaders such as these. The inverse is also true: Leaders like these can, and do, make a difference.

5. Independent activism is critical.
This shift in Maryland’s race can also be attributed to the hard work of independent activists. Issues-based groups have played an essential role in shifting the public debate. Other independent organizations are moving the “Overton window” through other means – by researching the issues, promoting issues-related activism, and/or supporting progressive political candidates.

6. Hillary Clinton and other national candidates face an opportunity – and a risk.
Politicians should remember: voters don’t forget. The Beltway’s “bipartisan” agenda is fundamentally unpopular, and those who promote it may have to defend their actions in the years to come.

While Hillary Clinton has yet to state her position on many key economic issues, President Clinton and a number of Clinton team members are closely associated with Simpson-Bowles and other unpopular “centrist” ideas.

The mood among the Clinton team seems to have shifted, at least somewhat, in recent months. Clinton associates and advisors are increasingly discussing wealth inequality and other populist-related issues. The (presumptive) candidate herself has also raised these themes in recent comments.

But voters have been burned, and are increasingly skeptical of promises that lack the force of commitment. They’ve heard progressive rhetoric from presidential candidates, only to see those candidates reverse themselves upon attaining the presidency. (There’s a plausible argument to be made that Bill Clinton, as well as Barack Obama, ran considerably more progressive campaigns than was later assumed in the “conventional wisdom.”)

Voters are likely to remain dubious about candidates who offer only vague platitudes about key issues like jobs, wages, and trade, without making firm commitments or offering specific proposals. The Maryland race has just begun, of course. But so far, it seems to point to the pitfalls of corporate “centrism” – and the promise of economic populism.