WASHINGTON - A broadly popular bill by Minnesota Democrat Amy Klobuchar to outlaw synthetic recreational drugs across the nation has run into an increasingly common obstacle in the U.S. Senate: the objection of a single senator.

Klobuchar and two other senators backing similar measures took the unusual step of taking to the Senate floor Wednesday to publicly denounce a "hold" placed on their bills by freshman Kentucky Republican Rand Paul, the son of GOP presidential contender Rep. Ron Paul.

"Let's hear what the objections are, and then pass these bills," said Klobuchar, who along with others, has been taking aim at a group of chemical compounds marketed as so-called bath salts, herbal incense and research chemicals.

Klobuchar was accompanied by Iowa Republican Chuck Grassley and New York Democrat Chuck Schumer, who have authored companion bills in the Senate. All three bills have been blocked from coming up for a final vote.

Despite a bipartisan coalition for anti-drug legislation -- a House version passed by wide margin in December -- Klobuchar and her allies find themselves bollixed by Paul, who by Senate tradition can single-handedly hold up legislation.

Paul spokeswoman Moira Bagley said he believes "law enforcement of most drug laws can and should be local and state issues." She also said that Paul's hold does not prevent Senate leaders from bringing the bills up for a vote, however it might be delayed.

Long-time congressional watchers say the tactic has been used with increasing frequency in recent years, particularly by Paul, a libertarian Republican who, like his father, is unafraid of challenging the Washington establishment.

Law enforcement officials in Minnesota and other states have been calling for congressional action to combat the growing epidemic of synthetic-party drugs like 2C-E, which is believed to be responsible for the death of a 19-year-old man in Blaine last year.

A Star Tribune investigation found that the drugs are often marketed as "legal" alternatives to illicit drugs like cocaine and methamphetamines. But they can have equally devastating consequences.

While the legislation has wide support in Congress, it also has its critics in civil liberties circles. One is U.S. Rep. Keith Ellison, D-Minn., who was among 82 Democrats and 16 Republicans who voted against the House bill. Ellison, as well as Paul, raised questions about the reach of federal drug laws and the exposure of low-level users, particularly minorities, to long prison terms.

Klobuchar said in an interview that if Paul has "philosophical objections" to her synthetic-drug bill he should not hold up a debate and vote. "We feel he should be able to speak to that and vote against it if he wants," she said. "But what we'd like to get from him, and hope we can get, is an agreement to simply have a set time for debate ... and then have a vote."

Schumer said he understands the right of a single senator to block a bill in the Senate, but not to foreclose debate. "Let's see if he can win people over to his point of view," he said.

Their predicament stems from the Senate tradition of "unanimous consent," which permits even a single senator to raise objections and push for a filibuster. Though Senate leaders can still bring the legislation up for a vote, it takes longer and endangers protocol.

While senators often complain about the holds, "they don't want to give up the power when they might be the one against the 99," said Norman Ornstein, a congressional scholar at the American Enterprise Institute.

'Death sentence'

As partisan tensions have risen in Washington, so too has the practice of placing holds -- often anonymously -- on bills and presidential nominations that individual senators don't like. But where holds were once generally confined to resolving scheduling conflicts or buying time for debate, "now they've become death sentences," Orn- stein said.

Klobuchar said she's talked to Paul and will continue to press the legislation with Grassley and Schumer, who provides clout as the No. 3 Democrat in the Senate. Senate passage of the three bills would send them to a conference committee to work out differences with the House-passed measure.

"They're close enough that we can work it out," Klobuchar said. "The basic premise is to put synthetic hallucinogens, synthetic marijuana and 'bath salts' on the list of prohibited substances. That makes it easier for prosecutors to prove up their cases."

More than 30 states, including Minnesota, have banned various synthetic drug compounds, but new ones emerge in a market that is spread nationwide through the Internet. Federal lawmakers have been working on a national response, trying to catch up with the spreading availability of the drugs, which Schumer called "as easy to buy as lollipops"

Recent statistics show one in nine high school seniors reports having tried synthetic pot; synthetic drugs are a rapidly growing cause for calls to poison control centers nationwide.

In particular, Klobuchar is targeting the drug called 2C-E. "This is a new type of drug, a very dangerous drug, and we have to act accordingly," she said.

Kevin Diaz is a correspondent in the Star Tribune Washington Bureau.