As we watch Bernie Sanders’ supporters struggling to come to terms with the nomination of Hillary Clinton, it makes sense to ask why leftists are involved in the Democratic Party in the first place.

It started in 1934 when Upton Sinclair, author of “The Jungle” and a socialist for most of his life, announced that he would run for governor of California as a Democrat. This began a unique relationship that has been important to American politics ever since.

Why unique?

In most countries throughout Europe and the Americas, the left has its own party or parties. And until the 1930s, American radicals were committed to standing apart from the two mainstream parties, especially the Democratic Party that had long represented white supremacy in the South and corrupt urban regimes in the North.

For 30 years, the Socialist Party carried the electoral hopes of most radicals. Then, in 1932, Socialist Party presidential candidate Norman Thomas endured a crushingly defeat, receiving just 2.2 percent of the vote.

Upton Sinclair, who had previously run for governor as a Socialist, now set out to do so again as a Democrat. His 1934 campaign electrified California and the nation. Announcing a bold socialistic plan to “End Poverty in California” during the Great Depression, he built a political movement much larger than anything the Socialist Party had ever accomplished.

I have written extensively about the EPIC movement and direct an online project that includes detailed accounts of the campaign and copies of campaign materials. And the Mapping American Social Movements project tracks the broader history of 20th-century radicalism.

Although finally defeated by red-baiting in the general election, Sinclair’s vote tally of 879,537 in California was close to what Norman Thomas had achieved nationwide.

The lesson was obvious. Radicals could do much better working inside the Democratic Party than trying to win elections on their own.

The New Deal Left

In the years that followed, radicals of many kinds became energetic New Dealers working from within to push the Democratic Party to the left. The Socialist Party withered. The radical labor activists who created the United Auto Workers and other new Congress of Industrial Organizations unions tied them closely to the party of Franklin D. Roosevelt. Even the Communist Party embraced the new strategy after 1936, still fielding some of its own candidates while working quietly to support progressive Democrats in what it called the “Democratic Front.”

In some states, radicals found ways to be both separate and included. The Minnesota Farmer-Labor Party and Wisconsin Progressive Party cooperated with Democrats at the national level while competing with the older party in state and local elections. The American Labor Party in New York was also formally separate but in practice endorsed progressive New Dealers.

In many other states, radicals created caucuses inside the Democratic party similar to what Tea Party activists have done recently in the Republican Party. Radicals in California briefly controlled the party apparatus and managed to nominate progressives like California Governor Culbert Olson and U.S. Representative Helen Gahagan Douglas and help them win elections until the late 1940s. The Washington Commonwealth Federation was still more effective. Operating as a formal organization within the Democratic Party, the WCF nominated slates of candidates, developed platforms and pressured lawmakers at all levels for progressive legislation.

Rocky from the start

So began the marriage between radicals and the Democratic Party that continues today. It has been rocky from the start and there have been several near-divorces as leftists at some moments retested the strategy of independence.

In 1948, as the Truman administration geared up Cold War policies at home and abroad, former Vice President Henry Wallace agreed to mount a third-party challenge. Supported by the Communists, Wallace failed to pull most leftists away from the Democratic Party. Truman won reelection and the left lost credibility. For the next two decades, the Democratic Party was decidedly centrist at all levels and almost every state.

The radicals who built new social movements in the 1960s around civil rights, black power, feminism, environmentalism and opposition to the Vietnam War had no tolerance for the centrist Democratic Party, especially after Lyndon Johnson guided the nation from cold to hot war. The alienation yielded a new third party, the Peace and Freedom Party, that secured a position on the ballot in several states in the contentious 1968 election. Mostly, however, the New Left shunned electoral politics in the late 1960s. Their revolution was taking place in the streets.

Then in the early 1970s, the marriage resumed. It started at local levels and had much to do with African-American activists mobilizing for municipal elections and with feminist campaigns to see more women in office. When George McGovern won the Democratic presidential nomination in 1972, he was carried along by millions of young people determined to end the war abroad and transform society at home. McGovern lost, but the activists reformed the party, rewriting nomination and convention rules in ways that would encourage grassroots activism and insure significant roles for women and communities of color.

Involvement and frustration

The framework of 1972 has given radicals ever since a stake in the Democratic Party. It’s also been the source of a lot of frustration. The role of the left is mostly invisible and thus different than 1930s and 1940s when clearly identified radical caucuses were common. For one thing, it is hard to know what “the left” is and who belongs to it. The contemporary left has no structure nor even a definite label. “Progressive” has become a vague identifier, but the term is used so loosely as to be almost meaningless.

Reuters/Mark Cardwell

Secondly, the left has been largely shut out of national level Democratic Party campaigns since 1972. Only once has there been anything like the Sanders campaign. In 1984, Jessie Jackson’s Rainbow Coalition primary challenge turned into a grand crusade that energized and expanded the left in a manner not unlike 2016. Otherwise centrists have commanded the party’s main stage.

More often radicals have been engaged in local-level campaigns where now and then an exciting progressive has been elected to office. Examples include Former Mayor Harold Washington in Chicago, Mayor Antonio Villaraigosa in Los Angeles and Mayor Bill de Blasio in New York. Black, Latino, Asian and gay candidates, ballot measures defending the rights of women, immigrants, and LGBTQ people, or helping working people — these are the campaigns that refresh the passions of progressives and keep them active in electoral politics.

Most of all it is the power and threat from the right that has kept radicals involved in the Democratic Party.

The Green Party was launched in 1990 to try an independent electoral strategy once again. After winning some city council seats in California and Wisconsin, the party mounted presidential campaigns behind Ralph Nader in 1996 and 2000.

When Nader’s Florida vote cost Al Gore the 2000 presidential election, the lesson of 1934 was reemphasized: whatever the attractions of third parties, the best hope of the left is inside the Democratic Party.

There is another lesson from 1934 that the Sanders team is probably thinking about. The relationships of the New Deal Era worked better than more recent versions because the left was organized and visible within the Democratic Party. Radical New Dealers dragged the party to the left, pushing policies that transformed the political economy of the nation and the rights of Americans. Will the revitalized left coming out of this election play a similar role in the years to come?