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“I think Ronald Reagan changed the trajectory of America in a way that Richard Nixon did not and in a way that Bill Clinton did not. He put us on a fundamentally different path because the country was ready for it.”

-- Then-Sen. Barack Obama in a January 2008 interview with the Reno (Nev.) Gazette-Journal.

Paul Ryan is the first member of a major party ticket to have come of age in the modern political era. He was 11 years old when Ronald Reagan first took the oath of office and subsequently steeped in politics and policies of the movement Reagan led.

President Obama, a decade Ryan’s senior, entered college with Jimmy Carter in the White House and was old enough to vote against Reagan in 1980.

Ryan wasn’t eligible to vote in a presidential election until 1992. By then, the Democratic Party had already transformed itself in response to three stinging defeats by Reagan and his vice president over old-guard Democrats.

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By then, Republicans, once divided by what George H.W. Bush called “voodoo economics” and a militaristic streak that concerned foreign policy pragmatists, had united behind the core tenants of Reagan’s doctrine: the economics of Milton Friedman, the foreign policy of “peace through strength” and a taut doctrine of social conservatism.

By the time Ryan was old enough to vote, the political landscape and the ideological discussion in America had been forever changed. “Liberal” was already considered an epithet to many Democrats and the Clinton-era of moderate “third-way,” center-left politics was in full flower.

Obama has devoted his political life to undoing what conservatives call the “Reagan Revolution” that changed politics in America and the president’s party. Reagan’s success took small-government conservatism, relegated to the musty nooks of the GOP since the end of World War II, into the political mainstream. Reagan made a formerly niche idea the dominant ideological driver in American public life.

Obama ran in 2008 against Hillary Clinton with the central attack that she and her husband were too moderate – too willing to accept the free-market, free-trade, trickle-down policies pushed by the right. Obama cast the Clinton era as a part of decades of dangerous rightward drift.

The liberal base of the Democratic Party, empowered by opposition to the Iraq war and amplified by the new connectivity of the Internet, agreed with Obama: Enough mollycoddling of conservatives. It was time to change the discussion.

Obama talked in the 2008 campaign about how he admired Reagan – not for his policies, but for his ability to reshape political reality. Obama aimed to be the Reagan of the left, the one who would return to the mainstream the liberal ideas that had been at the center of American political life from 1932 to 1980.

That did not happen. While Obama has retained much personal support from voters, his big ideas haven’t found much favor. Swing voters this election will be much more likely to vote for Obama in spite of his central policy achievements -- a huge stimulus package and a new government health-insurance program – than in spite of them

The best evidence of Obama’s inability to do for the left what Reagan had done for the right is in the tones of the incumbents’ campaigns in 1984 and 2012. In 1984, it was “Morning in America” in 2012 it is “Vampire.”

While Reagan embraced the idea of a referendum on his presidency, Obama has sought to make the election a referendum on Republican Mitt Romney’s character in the single nastiest campaign ever run by an incumbent of the modern media era.

Democrats blame the president’s embrace of personal attacks on the fact that the economy is still so lousy as a result of Republican policies in the previous decade that Obama has no choice but to ruin his opponent in order to prevent the return of those policies.

The logic on the left goes something like this: Had Gorge W. Bush not been president, Obama wouldn’t have been forced to run the kind of campaign he had previously denounced. In order to save the nation from conservatism, Obama is willing to break his own rules.

Implicit in that, though, is the promise that if Obama is given another turn in office, he can complete the transformation and return America to the time before 1980 when both parties embraced the palliative role of a large, muscular federal government and that higher taxes are not politically toxic.

The left is willing to wait for 2016 for a ratifying election on Obamaism, and for now is solidly behind the idea of the politics of personal destruction.

By selecting Ryan as his running mate, though, Romney is seeking to return to the paradigm for the election that Democrats had envisioned before they got their tails kicked in 2010: Reaganism vs. Obamism.

Ryan is in full the ideological heir to Reagan’s movement. Other national Republican figures have cast themselves as partners with Reagan (Newt Gingrich) or as a “foot soldier in the Reagan revolution” (John McCain), but Ryan’s career has been spent in a political world of Reagan’s making.

While former GOP entitlement-reform maven Jack Kemp might be Ryan’s ideological godfather, Ryan’s worldview was forged in the world of think tanks and conservative ideology that flowered in Washington post-Reagan.

This is significant right now because it is the strongest sign yet that Romney is ratifying Reagan’s worldview. Moderate Romney famously said in his 1994 senatorial campaign that he wasn’t a “Reagan-Bush” conservative. By taking Ryan, though, Romney pays homage to The Gipper in a way that has enlivened the GOP base and allayed some lingering doubts about how he would govern.

But going forward, the Ryan pick will be significant because it re-centers the discussion on whether the country will continue down the path it has followed in fits and starts since 1980 or whether liberalism will again be ascendant.

The president in Iowa today will announce that he is directing the Department of Agriculture to spend $170 million on a kind of quantitative easing for farmers by buying pork, lamb, chicken and catfish.   (Does Goldman Sachs even have a catfish unit?)

Obama will also be attacking Romney and Ryan’s plans for lower spending, lower taxes and less regulation: the Reaganite triumvirate.

Ryan will be in Iowa preaching the exact opposite and inviting the president into the world of policy attacks instead of personal ones.

As Reagan’s own ideological godfather, Barry Goldwater, said: “I will not change my beliefs to win votes. I will offer a choice, not an echo. This will not be an engagement of personalities. It will be in engagement of principles.”

What Romney and Ryan have to hope is that the political landscape of the Reagan era remains enough intact to prevent the fate that Goldwater met in 1964.

Chris Stirewalt is digital politics editor for Fox News, and his POWER PLAY column appears Monday-Friday on FoxNews.com.