Paul Starr is founding co-editor of the American Prospect and professor of sociology and public affairs at Princeton University.

If Democrats are going to deal with climate change, income inequality and other deep-seated problems, they don’t need only to unseat President Trump and regain control of both houses of Congress. They have to break the pattern of the past half-century.

Since 1968, Democrats have controlled both Congress and the White House three times, and each one of those periods ended with a hard turn right. Altogether, the years of unified Democratic government add up to just eight out of the past 52: four when Jimmy Carter was president, and the first two years of Bill Clinton’s and Barack Obama’s first terms. Carter’s presidency ended with Ronald Reagan’s election in 1980, Clinton’s first years with Newt Gingrich’s “Republican Revolution” in 1994, and Obama’s first years with the tea party insurgency in 2010.

This is the core problem for the party today: finding the leadership and policies that not only win in 2020 but also increase support instead of dampening it and igniting the opposition.

“Big, structural reforms,” to use Elizabeth Warren’s phrase, require sustained power. The federal government is riddled with “veto points” — opportunities for blocking change in Congress, the courts and the states that create a bias in favor of the status quo. The life tenure of Supreme Court justices and slow turnover of the Senate also put a brake on change.

Large-scale change requires Democrats to do what they did in the 1930s and 1960s and have been unable to do since — win a series of elections, build both popular and judicial majorities, and fundamentally alter not just individual policies but also the basic understanding of government’s role.

That’s why it’s a mistake to see the 2020 Democratic primaries as a stark choice between incremental moderates and transformational progressives. Democrats cannot create the necessary support for substantial, long-term change without incremental reforms achievable in the short term.

Consider climate reform, by its nature a decades-long project. Even with Democrats in charge, Congress is not going to enact a Green New Deal in 2021, but it could make a start, and the kind of start matters. Anyone interested in reducing carbon emissions ought to favor measures that reinforce political support for sustained emissions reductions. Clean-energy policies alone won’t be sustainable unless they’re combined with complementary investments, for example in resilient infrastructure, that bolster employment, help displaced workers and provide concrete evidence of achievement.

Or take health-care reform. Medicare-for-all is also not going to pass Congress in 2021, but Democrats could immediately reduce the age of eligibility for Medicare from 65 to 62 or 60, cut drug and other health-care prices, and improve coverage under the Affordable Care Act.

Using visible, incremental achievements to build support for larger ones is going to be crucial for making progress toward greater income equality. The needed structural changes in labor, antitrust and other areas are inherently long-term projects, but Democrats can immediately correct some of the economic imbalance with more equitable taxes and spending.

Sustained change requires, most of all, that a Democrat elected in 2020 have a successful presidency. Demand too much of Congress with proposals such as Medicare-for-all, and the result will be disappointment and electoral reverses. A Bernie Sanders presidency could well set in motion the same backlash that has undermined Democrats before. But aim too low in the hope of just getting back to a pre-Trump “normal,” and the result will also be deflating. Build on achievable reforms, offer an alternative vision of the future and bigger things become possible.

The two parties are in different positions. With their ambitious reform agenda, Democrats need continued, unified control of the government, while Republicans need only the power to block change. Even one more term for Trump would enable Republicans to consolidate control of the biggest veto point of all — the Supreme Court — whereas Democrats will likely need several terms to restore liberal influence not just on the Supreme Court but throughout the federal judiciary.

The Democrats’ challenge is also more difficult now that partisan differences have hardened. Under Dwight Eisenhower, Republicans made no effort to reverse the New Deal; under Richard M. Nixon, they accepted much of the change Democrats brought about in the 1960s. But today’s Republican leaders are intent on reversing Democratic initiatives, and even when they can’t, their judicial appointees might do it for them.

Perhaps the biggest item on the Democrats’ long-term agenda should be one that now seems the most inconceivable of all. They need to persuade Republicans to become partners on issues such as climate reform so that a rotation of power in Washington doesn’t threaten to undo desperately needed change. For now, though, Democrats must move ahead on their own, and to do it effectively they need leadership that bridges their divisions, and priorities strategically chosen to build support on the long road they have to travel.

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