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) -- Last month, Sen. Sherrod Brown of Ohio, a strong defender of the public option for health care, warned: "I don't want four Democratic senators dictating to the other 56 of us and to the country, when the public option has this much support, that it is not going to be in it."
But in the end, those senators won the battle over the public option, as well as several other provisions in the health care bill.
They have emerged as the powerhouses of the Senate.
Before 2009, most Americans never heard of Sens. Ben Nelson, Max Baucus or Kent Conrad. More have heard of the Independent Sen. Joe Lieberman as a result of his Democratic vice presidential bid in 2000.
Yet now they are the movers and shakers in the Senate. Anyone who follows politics knows exactly who they are. It could not be further from 2005 when Nicholas Confessore in The New York Times wrote that "centrist Democrats today struggle with an unfriendly environment."
Throughout the Senate debate over health care, the centrists repeatedly forced the president's hand by insisting on changes to the legislation that made Obama's liberal base furious and which will constrain the impact of this legislation. Health care was the second major victory for centrists this year. They also were able to cut down the size of the economic stimulus bill back in February 2008.
Why is this small group of senators so influential and will this change? The first reason has to do with the nature of the Democratic Party. Democrats have never been as ideologically disciplined as the Republicans, and they have been less successful containing party differences.
Moreover, since 2006, Democratic leaders embraced a campaign strategy of attempting to expand their numbers by encroaching into conservative districts and states: The 50-state strategy.
In 2006 and 2008, the strategy paid off. The cost, however, has been that President Obama needs to maintain support among legislators who come from areas that don't lean Democratic -- or who don't share his policy views.
When some Democrats pushed to punish Lieberman -- who supported Arizona Sen. John McCain in the 2008 presidential campaign -- by stripping him of his committee chairmanship, Obama refused and insisted on keeping him in the Democratic coalition.
The second factor behind the new kings of the hill has to do with the sharpening of the partisan divide on Capitol Hill. The impact of growing party polarization since the 1970s has meant that winning votes from the other party is extraordinarily difficult. Except for rare moments, neither party can count on winning significant blocks of votes from the other side of the aisle.
As a result, it is essential that the majority party remains relatively unified in order to pass legislation. Obama has learned this lesson well. With health care in the Senate, he even failed to win the vote of the moderate Republicans Susan Collins and Olympia Snowe. Because of such partisan intensity, the vote of just a few Democrats becomes that much more important. Nor can he afford to lose the vote of a defiant Independent, Lieberman, who still goes along with Democrats on many votes.
Finally, institutions matter. The U.S. now has a Senate that operates as a supermajority. The Senate now requires 60 votes on any piece of legislation given that senators are willing to use the filibuster on almost any bill. If the majority party needs 60 votes to pass a bill, and it can't win votes from the other side, a handful of moderates wield tremendous power.
With even less likelihood of winning any Republican votes on bills like climate change or a second economic stimulus bill, the president will have to continue to shape his agenda around these Democratic and Independent centrists in 2010 -- senators who are much different from the more liberal ones who drove Obama's support in the election and who will be crucial in 2012.
What can the president do? One option is be much tougher with these centrists, threatening to withhold party privileges and favors from those who don't play ball. This is a risky strategy since it could embolden the Democrats to challenge the president or switch parties.
The second option would be to use the power of the bully pulpit, as well as the famous but now forgotten campaign network from 2008, to go into the states of these senators and build grassroots pressure to vote with the party. Thus far, the president has been hesitant to take this path.
A final option would be filibuster reform. In 1975, the Senate lowered the number of votes required to end a filibuster from 2/3rd (67) to 3/5th (60) of the Senate. If fewer senators were required to end a filibuster, the conservative Democrats would instantly find themselves less relevant.
On PBS' NewsHour, Obama recently said, "As somebody who served in the Senate, who values the traditions of the Senate, who thinks that institution has been the world's greatest deliberative body, to see the filibuster rule, which imposes a 60-vote supermajority on legislation, to see that invoked on every piece of legislation, during the course of this year, is unheard of."
Changing the rules of cloture which govern when the Senate can end debate will be extraordinarily difficult since that vote, too, would ordinarily require 60 votes. But it has been done before. In 1975, Republican Vice President Nelson Rockefeller intervened in his role as the president of the Senate to allow for a majority to vote for reform.
If Obama and other Democrats are serious about making the Senate more responsive to a majority of its members, and if they are willing to push for filibuster reform, this would pose the most significant threat to the new kings of the hill.
Of course Democrats would have to take this step accepting the fact that when Republicans are in control of the Senate, conservatives would have a much easier time achieving their legislative priorities than they did under President George W. Bush.
The opinions expressed in this commentary are solely those of Julian Zelizer.