They’ve even produced their own flawed scientific studies supposedly proving that medical marijuana laws don’t increase use among teenagers, when almost all the evidence says just the opposite. How can Democrats criticize Republicans for disregarding science and making up facts when people on our side do the same?

Democrats know we need government regulation to protect the public from unhealthy products. But the marijuana lobby wants us to distrust two centerpieces of the regulatory state, the Food and Drug Administration and the Drug Enforcement Administration. The whole purpose of medical marijuana laws is to evade the regulatory power of these agencies. We’re the political party that got the F.D.A. to regulate tobacco. How can we now say it shouldn’t regulate pot?

Legalization also runs counter to the Democrats’ commitment to education. States with medical marijuana laws have always had much higher rates of teenage marijuana use, but now the effect is nationwide. Since 2008, teenage use has increased 40 percent, and heavy use (at least 20 times a month) is up 80 percent.

Blame the drive to legalize pot. It sends the message that weed is harmless, even though research shows that teenagers who use it regularly do worse in school, are twice as likely to drop out and earn less as adults. Teenage use has been shown to permanently lower I.Q.

No other drug, not even alcohol, affects academic performance like marijuana. How can we make education a focus, and then support laws that will blunt the next generation’s ability to compete?

Legalization would also undermine a successful Democratic program: drug courts, which were written into the 1994 crime bill by Senator Joseph R. Biden Jr. and signed into law by President Bill Clinton. They use coercion, the threat of jail, to keep addicts in treatment.

But the marijuana lobby opposes coercion. That’s not surprising. Drug users just want to be left alone to get high. If we side with them, we’re undercutting the Democratic answer to substance abuse.

In effect, America now has two tea parties: on the left they smoke their tea; on the right they throw it in Boston Harbor. Both distrust government, disregard science and make selfish demands that would undermine the public good. But while Republicans have completely caved in to their Tea Party, several Democrats, including the president, are standing up to ours.