Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University's Woodrow Wilson School. His new book is "Arsenal of Democracy: The Politics of National Security -- From World War II to the War on Terrorism" published by Basic Books. Zelizer writes widely about current events.
Princeton, New Jersey (CNN) -- Scott Brown's victory over Martha Coakley in Massachusetts has sent shock waves through the Democratic Party.
This is a devastating symbolic and practical loss for the party, one that turns the U.S. Senate seat of a liberal lion, the late Ted Kennedy, over to Republican hands. The loss drops the size of the Democratic majority down to 59, which is below the vaunted filibuster-proof majority.
This could very well just be a taste of things to come. Most likely, the midterm elections won't be good for the Democrats. Traditionally, midterms are not good for the party that controls the White House. With the exception of 1934, 1998, and 2002, since Reconstruction the president's party has suffered losses, with some worse than others, in the midterm that followed each president's election.
The current situation makes things even more troubling for Democrats. President Obama has energized conservative activists and concerned independents during his first year. The economy remains in poor condition with staggering rates of unemployment. And as a result of the compromises that Obama has made, as well as the escalation of the war in Afghanistan, many liberals are less enthused with the president.
Even if Democrats retain control of Congress in 2010, which still remains the most probable outcome, power within a Democratic Congress could shift significantly.
Unless conditions change dramatically, Democrats will likely experience midterm elections akin to 1966 or 1982, when the party controlling the White House maintained control of Congress but lost influence on Capitol Hill.
From the moment that he defeated Barry Goldwater in a landslide election in 1964, Lyndon Johnson was obsessed with the 1966 midterms. His fears caused him to resist proposals to withdraw from Vietnam, and to accelerate his efforts to obtain domestic legislation before his power diminished in Congress.
The midterms were almost as bad as Johnson feared. While Democrats maintained control of both chambers, the size of the conservative coalition -- the alliance of southern Democrats and Republicans -- had increased. Democrats lost four Senate seats and 47 seats in the House.
Republicans, a party some thought to be defunct after the devastating results in the 1964 election, seemed revitalized. The cover of Time magazine depicted six smiling Republicans. The story quoted Ray Bliss, chairman of the Republican National Committee, at a news briefing: "This press conference ... will be a little different from my first one, when you were asking me if the Republican Party would survive. ... It looks to me ... as if we have a live elephant."
House Majority Leader Carl Albert felt that the elections "broke the back of the Great Society right there." And as he predicted, Johnson had trouble moving any more domestic programs through Congress. -- though much of the Great Society legislation enacted before the midterms remained in force.
In 1982, two years after conservatives boasted of a Reagan revolution, with Republicans gaining control of the White House and Senate, Democrats increased their majority in the House. The terrible condition of the economy, with 10 percent of the workforce unemployed, had allowed Democrats to organize an effective campaign that focused on "fairness" and attacked the "Reagan recession."
Pollster Richard Beal warned in a White House memo that the elections were crucial and were "pivotal in American political history; it is crucial to sustaining the Reagan revolution beyond his first two years in office."
Democrats expanded their control by 26 seats in the House, as the conservative coalition diminished in size after having surged in 1980.
The newly elected Democrats opposed Reagan's efforts to cut domestic programs, voted for restrictions to prevent Reagan from providing assistance to anti-communist forces in Nicaragua, and supported a freeze on the production of nuclear weapons. With the new division of power in the House, the president found it extraordinarily difficult to obtain support in Congress.
If President Obama suffers through a similar kind of midterm experience, he will have to deal with a Congress where his opponents have enough votes to force even bigger compromises than this year, thus angering liberals, or to block progress on his agenda altogether. That process has already begun as a result of Massachusetts, and now the White House must do everything possible to make sure that the situation does not get even worse in November.
The opinions expressed in this commentary are solely those of Julian Zelizer.