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Hours after the Boston Marathon bombings, President Barack Obama gave the standard presidential line following a tragedy: "On days like this there are no Republicans or Democrats — we are Americans, united in concern for our fellow citizens."

And, as usual, Republicans and Democrats alike quickly ignored his don't-politicize-this plea.

This was inevitable.

Our leaders always play politics after catastrophe, whether made by man or Mother Nature. The Newtown shootings and Superstorm Sandy. The financial crisis and Hurricane Katrina. Our history is filled with moments when something big happens and elected officials maneuver quickly to take advantage of the changing public mindset — or at least the more intense media spotlight — on a specific issue.

Democrat Franklin D. Roosevelt and Congress leveraged public angst over the Depression and a worldwide war in the 1930s to enact the New Deal, overhauling financial systems, funding public works projects and creating Social Security. Some three decades later, Lyndon B. Johnson and his Democrats seized on social unrest to pass the Great Society, anti-poverty and civil rights measures, education and transportation initiatives, Medicare and Medicaid.

During the 1980s, Ronald Reagan and his GOP used the moment of sky-high inflation and a growing Soviet threat to win support for boosting the military, trimming government and cutting taxes. And, in the aftermath of the 9/11 attacks, Republican George W. Bush rallied a fearful America behind expanding the government's terrorist-tracking powers, streamlining intelligence gathering and toppling Saddam Hussein.

Most recently, when he took office amid the worst economic conditions in a generation, Obama saw an opportunity to advance an audacious agenda that included ending the costly war in Iraq, improving crumbling transportation arteries and overhauling the health care system. As his first chief of staff, Rahm Emanuel, was fond of saying back then: "You never want a serious crisis to go to waste."

A gray area exists in all cases.

To some people, politicians who press for new legislation after a tragedy are seizing the perfect time to make needed changes, using typically fleeting we-are-one moments to reach consensus on an issue that long had been languishing behind more pressing priorities or struggling to get the necessary votes. To other people, these politicians are exploiting a tragedy in a blatant attempt to enact their pet, partisan policies.

These days, Republicans and Democrats alike accuse each other of politicizing tragedy — even as they do the same. And, in this season of political gridlock, both parties typically use such moments to dig in on their polarizing positions, rally their core supporters and pressure the public to see it their respective ways.

In doing so, they waste opportunities to find common ground to address a problem that a calamity illuminated, like the failed effort by a bipartisan group of senators to close gun law loopholes in the aftermath of the Sandy Hook Elementary School shooting in Connecticut. They couldn't persuade enough lawmakers to buck their core backers and campaign donors in the name of compromise.

And when today's lawmakers do find solutions in the midst of crisis, those accords usually are forged at the last minute and only after intense partisan wrangling, such as when the country was about to head over the so-called "fiscal cliff" of automatic tax increases and budget cuts.

Blame the far wings of the political parties that control the system; blame a media that feeds on conflict; blame special interests that threaten to work to fire lawmakers who capitulate.

And, to some extent, blame the reactive nature of Congress. It doesn't do a good job anticipating problems, and rather appears to be in a constant crisis management mode, dealing with the biggest issues only when they demand the most attention — and making them catnip for partisans looking to push their pet positions just when public opinion is changing.

This has been the case in the seemingly never-ending cycle of fiscal emergencies recently.

Tom Daschle, the former Senate Democratic leader from South Dakota, sees parallels between Congress and a fire department that responds to five-alarm blazes but doesn't focus enough on prevention, saying, "We're responding to the fires because, partly, that's where the media is, that's where the people are demanding that we respond to these things."

He says it's difficult to reach common ground proactively, adding: "It may even be impossible in some cases because they don't have the pressure to reach that consensus. So you often times kick around ideas but never really get to a critical tipping point to bring about the change required."

Tragedy creates such a tipping point — and triggers the political maneuvering.

This, of course, is not always a bad thing, and it can result in quick attention to problems that long have gone unaddressed. Sometimes, this is how the big stuff gets done in our country.

Think back to the government money that poured into the Gulf Coast to fix inadequate levees in the aftermath of Hurricane Katrina. Or the reining in of an under-regulated Wall Street after the housing bubble burst, sending the nation into recession. Or, following the 9/11 attacks, the realignment of America's national security sectors, which had a history of not communicating with one another.

So it was to be expected that within hours of the Boston bombings, Democrats and Republicans would seek to use the tragedy to their advantage on a host of issues.

Politicians on Twitter referenced the bombings to make points — and press their positions — on everything from guns to torture, illustrating how seemingly separate issues get intertwined as all sides seize on tragedy to score political points.

The most high-profile example occurred during the start of Senate debate over legislation to remake the U.S. immigration system. Republicans argued that the role of two immigrant suspects in bombing raised questions about gaps in the system even though there was no suggestion that the two suspects, brothers from Russia, had entered the U.S. illegally.

"Given the events of this week, it's important for us to understand the gaps and loopholes in our immigration system," Sen. Chuck Grassley, R-Iowa, the Judiciary Committee's senior Republican, said as he began a hearing. That prompted this from Sen. Chuck Schumer, D-N.Y.: "I'd like to ask that all of us not jump to conclusions regarding the events in Boston or try to conflate those events with this legislation."

Of course, there's a flip side.

It's a good bet that the Boston bombings will prompt local officials and state lawmakers to ensure that law enforcement officers and first responders have the resources they need should terrorism come to their towns. It's also likely that the United States will use this moment to continue to refine its terrorist-tracking methods. And, if terrorism becomes more important to the public than it is now, it's possible that lawmakers could be compelled to act on any number of national security matters that have been languishing.

If that happens, this may end up being the lesson we draw: That playing politics not only is inevitable — but it's also necessary to govern, particularly in an era of stalemate.

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EDITOR'S NOTE — Liz Sidoti is the national politics editor for The Associated Press. Follow her on Twitter: http://twitter.com/lsidoti