Three U.S. senators joined forces in March on a bill that would

legalize medical marijuana under federal law.

The CARERS Act, proposed by Sens. Rand Paul (R-Ky.), Cory Booker (D-N.J.), and Kirsten Gillibrand (D-N.Y.), would also change marijuana from a Schedule I substance to a Schedule II substance under the Controlled Substances Act.

Though some experts say the bill probably won't pass, it could be successful in making a statement.

"The bipartisan senators that are coming to the bill are probably dubious of the bill's passage, let alone to achieve even a lowly sub-committee hearing to review/debate the bill," said Allen St. Pierre, executive director of the National Organization for the Reform of Marijuana Laws. "But, they recognize that members of the House have been regularly introducing cannabis law reform bills, and that the Senate, too, should engage in legislation that reflects the changes in the country's attitude rejecting cannabis prohibition."

The bill could also lay the groundwork for future policy reform.

"I think the CARERS Act could pass the Senate Judiciary Committee if it came up for a vote," said Bill Piper, director of the Drug Policy Alliance's office of national affairs. "However, the Republican chair of the Judiciary Committee, Chuck Grassley, says he opposes the bill. We’re going to try to put pressure on him to bring the bill up anyway."

"Grassley has also said he supports more incremental reform -- so this bill might make less ambitious reform possible," Piper added.

The fact that the bill was proposed by Booker and Gillibrand, two Democrats, and Paul, a Republican, shows how the legalization movement transcends party lines, said Marijuana Policy Project Communications Manager Morgan Fox.

"Marijuana policy reform has always been supported by conservative ideals like states' rights, limited government, fiscal responsibility, and personal liberty," Fox said. "The increasing bipartisan support simply means that lawmakers are less afraid to support reform now that a majority of Americans agree that it is time to end marijuana prohibition."

The bill gathered more support after its proposal, with Sens. Dean Heller (R-Nev.) and Barbara Boxer (D-Calif.) coming on board to cosponsor it.Partisan differences could still stymie legalization progress, however.

"Such a long and deep-seated public policy like the federal government's cannabis prohibition, created with bipartisanship in the 1930s, must too end with bipartisanship," St. Pierre said. The challenge for reform activists and policymakers is that there is about a 20 percent difference between a majority of Democrats supporting legalization -- 55 percent of Democrats support legalization -- and a majority of Republicans favoring the status quo of prohibition. [Only] 35 percent of Republicans support legalization."

With Rand Paul preparing to announce a 2016 presidential bid, coming out in support of legalization could prove a shrewd way to reach certain demographics.

"Polls show a majority of Americans support legalization," Piper said. "A super-majority supports legalizing medical marijuana. Legalization is especially popular among young people and independent voters. Any presidential candidate that embraces marijuana law reform is going to get more votes than they lose."