Social Security is a big problem for Hillary Clinton.

It's an issue that is forcing her to choose what kind of Democratic leader she wants to be: one who is willing to compromise with Republicans in the interest of progress, or one who will firmly stand by progressive principles. Clinton can't avoid this dilemma. Social Security is a real pocketbook issue for millions of Americans, and her opponent, Sen. Bernie Sanders, is demanding that she take a clear position.

Social Security is running out of money, and it will have to stop paying beneficiaries in full in less than 20 years, projections indicate. On the other hand, the Democratic base wants to see the program expanded. They feel that benefits aren't generous enough, and that many elderly Americans who are living on Social Security can't get by.

In the past week, Clinton has made clear that she is choosing principle over compromise, taking reductions in benefits off the bargaining table and talking in Thursday's debate about expanding benefits for vulnerable groups.

To be sure, activists won't be satisfied with her position, and Sanders will continue to criticize her as insufficiently liberal on the issue. Despite that, and although she has cast herself as the pragmatic, sensible alternative who could get things done in the White House, Clinton is actually running as a progressive, liberal Democrat, not a dealmaker.

That Clinton would take this approach was not certain at the beginning of the campaign, but she has shifted to the left -- not only on Social Security but also on trade and other issues, reflecting the increasing clout of liberal groups within the Democratic Party.

In her first campaign for president, for example, Clinton suggested that to deal with Social Security's shortfall, the parties would have to compromise. "Let's put together a bipartisan commission and look at how we're going to deal with these long-term challenges," she said at a debate in 2007. While she made clear that she personally did not think Social Security should make up its deficit by decreasing benefits for retirees or increasing taxes on ordinary workers, any bipartisan commission would presumably recommend some combination of the two.

On social media last week, though, Clinton took an unequivocal stance in response to a goad from the Vermont senator. She promised to "defend" and "expand" Social Security.

She followed up in Thursday's debate at the University of Wisconsin in Milwaukee with a detailed discussion of the issue.

"I want to start by helping those people who are most at risk, the ones who, yes, are cutting their pills in half, who don't believe they can make the rent, who are worried about what comes next for them," she said. She argued that widows and women who have spent many years out of the work force caring for loved ones in particular are likely to be short on cash.

Her approach is different from that of Sanders, who wants to make Social Security more generous for all retirees.

The Social Security program's actuaries issued new projections for Sanders's plan last week. By substantially raising taxes on the wealthy, Sanders would both increase benefits for poor and working-class retirees while improving the program's overall finances.

The actuaries found that under Sanders's plan, a low-income retiree who works 30 years and earns an average of $22,000 a year in 2015 dollars would receive 18 percent more from Social Security. The program would be able to continue paying benefits in full for another four decades -- although in the long term, Sanders's plan isn't fiscally sustainable either.

Clinton hasn't put forward a formal proposal, but a document from her campaign states that she would oppose increasing the retirement age, reducing the annual cost-of-living adjustment or privatizing the system -- typical Republican ideas for eliminating the shortfall. That doesn't leave much room for compromise.

Clinton's subtle shift in tone on Social Security is a major victory for liberal bloggers and organizations that have advocated for expanding the program, such as the Progressive Change Campaign Committee and Social Security Works. It is just one sign that the Democratic Party has become more amenable to government intervention in the economy.

The party's liberal wing still isn't satisfied with Clinton. They want her to make more ambitious proposals on Social Security, health care and more. In the context of the broad range of views the party represents, however, Clinton is driving squarely in the left lane.

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