In recent American political history, changes in political momentum typically revolve around a seminal political battle.

After the Republican sweep in 1994, that battle was over the GOP plan to cut Medicare to provide tax cuts for the rich. It featured Newt Gingrich’s government shutdown and his subsequent retreat in 1995. From that point forward, Clinton built momentum and ultimately defeated the Republican nominee Bob Dole by 8.5 percentage points.

A similar decisive battle turned the tide ten years later, after the Republican victory in 2004. In the months following their defeat, Democratic prospects looked bleak. Republicans controlled the Senate, House and the Presidency and were poised to seize control of the Supreme Court for a generation.

But then Bush and his Wall Street allies launched a massive effort to privatize Social Security — a move designed both to eviscerate the social insurance program that lay at the foundation of the New Deal and to allow Wall Street to get its hands on the Social Security Trust fund. President Bush toured the country to stump for his plan, the Republican leadership signed on in support.

Democrats stood solidly against the proposal and together — with the labor movement and other progressive organizations — ran a campaign that ultimately forced the Republicans to drop the proposal without even so much as a vote in Congress. It turned out that privatizing Social Security — which would have simultaneously lowered guaranteed benefits, and increased the deficit — had zero traction with ordinary voters who believed that the money they had paid into Social Security entitled them to the promised guaranteed benefits.

The battle to privatize Social Security shifted the political momentum in America. Democrats got back off the floor after being thrashed in 2004, regained their footing and self-confidence and went on the offense — attacking the increasingly unpopular War in Iraq and capitalizing on the unbelievable incompetence surrounding Hurricane Katrina. After Democrats took control of the House and Senate in 2006, that momentum continued through Barack Obama’s victory in 2008.

After their defeat in 2008, Republicans used the battle over health care reform to turn the political tide themselves. They didn’t win the fight over the health care bill, but they won the political war. They used that momentum to invigorate their base and to capitalize on the slow pace of economic recovery after the financial catastrophe that was actually caused by reckless Republican economic policies coupled with wild excesses on Wall Street.

Politics is like war — or for that matter competitive sport. Momentum is critical to victory and changes in momentum inevitably center on turning-point battles. Just as important, turning-point battles reframe the terms of debate. They become emblematic of whether or not a political leader is “on your side.”

Political momentum shifts have an enormous effect on political psychology. For one thing, there is the band-wagon effect. People don’t like to sign on with losers — or political parties that are despondent and divided. Voters, candidates and donors, want to be with self-confident winners — not losers who are searching for direction. They get on the train when it’s picking up steam — not when it is grinding to a halt.

That’s why the perception that political momentum has changed can often become a self-fulfilling prophecy.

Regaining the political momentum will do wonders for Democratic attempts to raise funds and recruit candidates for the elections in 2012. It has already encouraged several of the strongest contenders in the Republican presidential field to take a pass on the race.

And without iconic battles, momentum shifts in politics rarely occur.

After George Bush won the Presidency in 2000, the battle over the Bush tax cuts could have taken on that kind of iconic importance. Unfortunately, even though Democrats could have stopped his tax cuts for the wealthy, much as they stopped his attempts to privatize Social Security after 2004, some Democrats did not hold firm and draw a line in the sand. A few Democrats joined the Republicans to support the Bush tax cuts that have led directly to our current budget deficit. Their success passing tax cuts for the wealthy built momentum for the Republicans.

And, of course, there was another iconic moment that most defined the first years of the Bush Presidency: the attack on 9/11. The Republicans used that attack as a huge political momentum builder, and it served as the rationale for almost all of their policies for the next four years.

By proposing to eliminate Medicare, Republican Budget Committee chair Congressman Paul Ryan set the stage for exactly the kind of iconic battle that signaled fundamental changes in political momentum in the past. Over the last six weeks, that battle has played out in town meetings and talk shows across the country. It culminated last week in the stunning Democratic victory in New York’s blood-red 26th Congressional District, where it became crystal clear to everyone that the Republican plan to eliminate Medicare is a political kiss of death.

The fact that Ryan and the Republicans chose political low ground to engage this battle is not entirely a result of Republican hubris or dumb luck. David Plouffe and the Obama team deliberately laid in wait for the Republicans, holding back at engaging the budget debate until Ryan and company made their incredibly unpopular proposal — and then the President’s budget speech sprung the trap.

They knew that once the Republicans had elaborated their strategy to eliminate Medicare in gory detail they could demonstrate graphically just what America would look like if the Republican ideologues had their way.

Amazingly, this weekend, Republican leaders doubled down on their proposal, pledging to make it part of the terms Republicans will demand to avoid default of America’s debts.

Apparently the Republican leadership’s desperate need to pander to the extremist Tea Party element in their ranks has overwhelmed their good political sense – and that is great news for Democrats.

The battle over Medicare — and the entire Republican budget — puts the question of “who’s on whose side” in clear, unmistakable relief. As in 1995, the issue is simple. In their budget, Republicans proposed to cut – actually eliminate – Medicare in order to give tax breaks to millionaires.

During the 2005 battle over privatizing Social Security, the Republican leaders never even came close to actually forcing their Members to cast a vote to support Bush’s radioactive privatization plan — yet the battle still turned the political tide. This year, the Republicans were so cowed by the Tea Party that they actually corralled all but four Republican House Members — as well as forty Republican Senators — into voting yes on a bill to eliminate Medicare. Astounding.

The decisive battle that has changed the political momentum between the conservative and progressive forces in American society has happened — and once again Progressives have stood up straight and are on the march.

Now we must press our advantage and use this iconic engagement to demonstrate clearly that the radical conservatives are a wholly-owned subsidiary of the CEO/Wall Street class – the wealthiest two percent of Americans — while Democrats and Progressives stand squarely with the middle class.

Robert Creamer is a long-time political organizer and strategist, and author of the book: Stand Up Straight: How Progressives Can Win, available on Amazon.com. He is a partner in the firm Democracy Partners. Follow him on Twitter @rbcreamer.

Follow Robert Creamer on Twitter: www.twitter.com/rbcreamer