From 2000 to 2008, no one did more to unite the Democratic Party than George W. Bush. But Democrats haven't fallen apart in the years since Bush exited the political stage. Instead, President Barack Obama has maintained their cohesion thanks in large part to his personal popularity with the base, unrelenting Republican opposition and strong Democratic congressional leadership. But simmering beneath the surface of this united front is an ascendant progressive and populist movement that is on the verge of taking over the party.

The very forces that have held the Democratic Party together are forestalling the takeover. Many of Obama's most enthusiastic supporters have been loath to criticize him publicly—even when the president's idea of "change you can believe in" hasn't matched up with his campaign promises. As long as the party is defined and controlled by the sitting president, these progressive impulses will remain somewhat muted.

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The lead-in to the 2016 presidential campaign could force a tipping point as early as next year if Hillary Clinton declines to run and a broad field emerges. If that happens, candidates will feel a great deal of pressure to appeal to the highly engaged, energized and well-funded activists who have been clamoring for a robust progressive agenda. Even if Clinton runs, her candidacy won't preempt the party's eventual takeover by the activist forces. It will only slow it down. Candidate Clinton, who appears to have the overwhelming support of the activist base, will nevertheless feel pressure from the left to pursue a more economically populist approach to solving our country's problems.

And now, with the left lining up around a Clinton candidacy, the activist base will continue to make incremental progress toward assuming control of the Democratic Party. Absent any countervailing forces that have yet to emerge, there won't be the same kind of intra-party battles between liberals and moderates that took place in the 1970s and 80s. Those conflicts were finally resolved in the '90s when Bill Clinton brought together the competing forces that had divided the Democrats and alienated swing voters since the 1960s, largely by focusing on improving the lives of the middle class while not betraying the core values of the party.

But Hillary Clinton will take control of a different party than her husband did because Democratic activists and elected officials have only become more liberal over the last two decades. Last month, the Pew Research Center released the findings of one of its largest studies of U.S. political attitudes ever undertaken. The 10,013-person survey quantified the increase in the share of Democrats holding mostly or consistently liberal views over the last 20 years. In 1994, only 30 percent of Democrats considered themselves mostly or consistently liberal, but this number increased sharply to 56 percent of respondents in 2014.

The study also found that 70 percent of politically engaged Democrats are mostly or consistently liberal in their views—double the 35 percent that fell into this category 20 years ago.

Several factors are driving this transformation, including the country's changing demographics and growing concerns about a disappearing middle class and stagnant social mobility. Wealthy liberal donors are accelerating the change, as are grassroots activists who rely increasingly on new technologies to expand their ranks.

These progressive forces are coalescing around a populist-inspired desire to combat income inequality and rein in large financial institutions, as well as an interest in focusing on priorities at home rather than abroad. It's difficult, in this environment, to imagine a viable Democratic presidential candidate who isn't willing to take clear positions on issues like increasing the minimum wage, securing comprehensive immigration reform, supporting women's health and their reproductive rights, addressing climate change and eliminating or at least curtailing fracking.

(See Doug Sosnik's other political memos: Which Side of the Barricade Are You On? and Groucho Marx's Republican Party)

The left's rise is aided by the fact that it is more organized than ever before. Following George W. Bush's defeat of John Kerry in 2004, a coalition of progressives from politics, philanthropy and business came together to build a long-term infrastructure—independent of the Democratic Party—to advance their progressive agenda and beat back the influence of the right wing. The Democracy Alliance was officially launched in 2005 as a forum where partners who shared core progressive values could coordinate their resources more efficiently to advance their common goals. Politico estimates that Democracy Alliance-backed groups plan to spend $374 million this election cycle.

Doug Sosnik is a Democratic political strategist. He was formerly political director in President Bill Clinton's White House.