Why did America change its mind about legal marijuana?

A new poll says that 51 percent of Americans support legal marijuana for recreational use. This comes a month after two states became the first to do just that. 

One month after Colorado and Washington State voters approved legalized marijuana for recreational use, a new poll suggests that a majority of Americans back the move. 

According to a Quinnipiac Poll released Wednesday, 51 percent of voters support legalizing pot for recreational use, with 44 percent opposed. The poll continues an upward trend of acceptance, with legal pot breaking the 50 percent threshold in a Gallup poll last year. In 1969, support was only 12 percent.

The trend will likely test Congress. For now, federal law puts marijuana in the same class as heroin and cocaine, but evolving national opinion as well as the growing patchwork of recreational and medical marijuana laws in states nationwide could pressure Washington into a more nuanced position.

The dramatic change in public opinion, experts say, has been driven by pop culture and generational shifts, and also a simple reality. While pot is illegal, it is common at parties and concerts. And despite warnings from anti-drug warriors, its impact on society has not been seen as disastrous, they say. In other words, American's don't associate pot with images of junkies zonked out on street corners.

"If marijuana ever became completely legal, then it is likely that more people would use … [and] it is also likely that some ill effects would occur from that use," writes Lance Dodes in Psychology Today. But "it is unlikely to catalyze a major shift away from current addictive use of alcohol or other compulsive behaviors, and it is unlikely that the total number of people with addictions will rise significantly."

In the eyes of many Americans, particularly in younger generations, the drug is now seen as being in the same class as beer. Some 69 percent of voters under age 29 support legalization, according to Quinnipiac.

"With the use of marijuana for medicinal purposes legal in about 20 states, and Washington and Colorado voting this November to legalize the drug for recreational use, American voters seem to have a more favorable opinion about this once-dreaded drug," Peter Brown, assistant director of the Quinnipiac University Polling Institute, told CBS News. "There are large differences on this question among the American people."

Though boosted by successes in Colorado and Washington, pro-marijuana advocates say their toughest challenge is convincing Congress and President Obama to declassify pot as a Schedule 1 drug – or at least to ensure that Congress doesn't interfere with state experimentation on marijuana taxation. After all, Mr. Obama allowed his Justice Department to begin a crackdown on medical marijuana dispensaries despite a campaign promise to not do so.

“I can’t nullify congressional law," Obama explained in a Rolling Stone interview this summer. "I can’t ask the Justice Department to say, ‘Ignore completely a federal law that’s on the books.’ What I can say is, ‘Use your prosecutorial discretion and properly prioritize your resources to go after things that are really doing folks damage.’ As a consequence, there haven’t been prosecutions of users of marijuana for medical purposes.”

Congress signaled an unwillingness to change its stance this summer, when the House voted 262 to 163 to defeat a budget amendment aimed at barring the use of taxpayer money to prosecute medical marijuana cases.

Yet both Republicans and Democrats have found political footing on the pot issue in states. Chicago Mayor Rahm Emmanuel (D) switched his views on pot this summer by backing a decriminalization bill that was ultimately approved by the city council. And Republican lawmakers in New Hampshire led an effort to allow marijuana use for medical purposes, though the state's Democratic governor, John Lynch, vetoed the bill.

Such efforts, however, suggest that some "politicians and their political parties are finally beginning to embrace the potential power of the pro-pot vote," writes Paul Armentano on the progressive AlterNet website.