WASHINGTON — Just five months after Washington state and Colorado voted to legalize marijuana for recreational use, a poll released Thursday found that a majority of Americans now say it should not be illegal to smoke the drug.

And, as Attorney General Eric Holder tries to figure out how to respond to the new legalization laws, the poll found that 60 percent of Americans say the U.S. government should not enforce federal drug laws in any state that has voted to legalize pot.

The poll found a strong consensus among people of all political persuasions for the federal government not to intervene: 64 percent of those who identified as independents, 59 percent of Democrats and 57 percent of Republicans.

Overall, 52 percent of Americans now say marijuana should be legal, while 45 percent say it should remain illegal, according to the poll conducted in mid-March by the Pew Research Center.

The center said the results marked the first time in more than four decades of polling on the issue that legalized marijuana had won majority support.

More Americans are experimenting with marijuana, too, the poll found: 40 percent said they had smoked the drug, compared with 38 percent a decade ago. And the poll found a sharp decline in the percentage of Americans who now believe that marijuana is a “gateway” drug that leads users to try other, harder drugs such as cocaine.

The poll found that 50 percent of all baby boomers now back legalized marijuana, compared with only 17 percent of boomers in 1990.

But it’s mainly younger Americans who are propelling the drive to legalize the drug, with 65 percent of adults born since 1980 – now between the ages of 18 and 32 – known collectively as the Millennial Generation, backing the idea. That compares with just 36 percent for the same age group five years ago.

The poll found that support for legalizing marijuana rose by 11 points among all age groups since 2010. That’s a huge change since a 1969 Gallup survey found only 12 percent backing legalized marijuana, while 84 percent were opposed.

Another Gallup poll in December found 48 percent of Americans backing legalized pot, while 50 percent were opposed to the idea.

Reacting to Thursday’s Pew poll, Steve Fox, national political director for the Marijuana Policy Project in Washington, D.C., said marijuana prohibition “is a policy without a rational basis and without public support.”

“These results do not just represent a tipping point,” he said. With support for legalization among people under age 50 at close to 60 percent, he said, “This is more like the tip of the iceberg. Elected officials across the country need to listen to the people.”