AUGUSTA, Maine — The Maine Senate on Tuesday gave its initial approval to a bill allowing public charter schools in Maine.

In a 21-13 vote, senators signed off on LD 1553. If finally adopted by the House and Senate and signed into law by Gov. Paul LePage, who supports charter schools, the bill would make Maine one of 41 states to allow the publicly funded alternative schools.

The vote came after an hour-long debate during which supporters called the introduction of charter schools a necessary step to help meet the needs of all Maine students and give their parents an alternative to the existing public school system.

“You can’t pound a square peg into a round hole,” said Sen. Garrett Mason, a Lisbon Falls Republican and the bill’s sponsor. “We have to help these students where they are.”

Mason offered an example from his largely rural district in which children aspiring to take over their family farms might benefit from a charter school that specializes in agriculture, rather than find themselves in a traditional high school that focuses on preparing students for college.

The bill would allow the creation of up to 10 charter schools in Maine over the course of 10 years starting in July 2012.

Charter schools are publicly funded educational institutions that allow more flexibility in terms of  curriculum and schedule, but still are held to state and federal educational standards.

While a recent poll showed more than 65 percent of Mainers in favor of charter schools, the plan was not without its opponents Tuesday as several senators questioned the need for — and effectiveness of — the alternative schools.

“The results are not glamorous,” said Sen. Justin Alfond, a Portland Democrat, citing the performance of charter schools in other states. Alfond said 37 percent of charters perform worse than public schools, and 46 percent produce results on par with public schools. That leaves only about 20 percent of charter schools that perform better than their public counterparts.

Additionally, Maine’s 82 percent high school graduation rate is far above those of many other states with charter schools. In Texas, there are about 100,000 students in that state’s 390 charter schools. The graduation rate there is 65 percent. In Florida, where more than 137,000 students are enrolled in the state’s nearly 400 charter schools, the graduation rate is just 58 percent, Alfond said.

“I think we truly are taking a big leap of faith here,” Alfond said of the chances that charter schools would improve education in Maine.

Alfond and other opponents also raised concerns that the new system would siphon the best students — and increasingly scarce funding — from local public schools.

In order to open, a public charter school must win the approval of a newly formed seven-member state charter commission. Under the legislation, the per-pupil allocations that towns spend on each child would follow the child to the school of his or her choice.

“Public schools will have more and more difficulty maintaining programs as students leave,” said Sen. Elizabeth Schneider, an Orono Democrat, who called the trend a “massive concern.”

“[Charter schools] are not the answer,” Schneider said, noting that the state has yet to reach the mandated 55 percent funding level for public schools. “The answer is funding our public schools.”

Although LD 1553 appears poised for passage this legislative session, Maine lawmakers have been cool to charter schools in the past, rejecting similar legislation on 17 previous occasions.

Tuesday’s Senate vote was largely along party lines with Republicans supporting the bill and Democrats opposing it. But there were some exceptions. Among them, Republican Sens. Nichi Farnham of Bangor and Roger Sherman of Houlton opposed the bill, and Democratic Sens. Joseph Brannigan of Portland and Nancy Sullivan of Biddeford supported it.