DOVER — Lawmakers will attempt to overturn a gubernatorial veto for the first time in 39 years today.

After contentious debate, and two trips through each chamber, legislation allowing parents to opt their children out of standardized school tests passed in the spring. It was vetoed by Gov. Jack Markell in July.

Main sponsor Rep. John Kowalko, D-Newark, is leading the push to overturn the veto.

“This bill is about parental rights, and of course, children’s rights,” he said in a Wednesday letter to colleagues. “Children have a right not to be fearful. Children have a right not to be humiliated. Children have a right to go to school and be in a continuous learning environment.

Rep. John Kowalko

“This bill ensures those rights. It says children may be excused from the test and engage in an alternative learning experience while others are taking the test.”

Parents already are technically not restricted from opting their children out, but supporters say the bill would codify that.

The proposal, House Bill 50, has received support from both sides of the aisle, as well as a vocal group of parents and educators who feel the Smarter Balanced Assessment is too challenging and has little use in measuring students and teachers.

The administration has argued the state risks losing federal funding if enough students do not take the test.

“HB 50 would undermine the only objective tool we have to understand whether our children are learning and our schools are improving,” Gov. Markell said in a July statement explaining his veto.

“It has the potential to marginalize our highest need students, threaten tens of millions of dollars of federal funding and undermine our state’s economic competitiveness — all without adequately addressing the issues that motivated many to support the legislation.

“That is why educators and school leaders have joined the civil rights community and business leaders in opposing the legislation, and why I am returning the bill unsigned.”

The bill was his 14th veto in his seven years in office.

Rep. Kowalko will first attempt to suspend the rules to bring the bill to the floor, an action requiring three-fifths support. If he is successful, the House can then vote on overturning the veto. Again, three-fifths is needed for it to pass the House, at which point it would go to the Senate.

Legislators have expressed uncertainty as to the chances the attempt will gain the necessary number of votes.

While the bill passed both chambers in a veto-proof majority last year, it is a virtual certainty the margin will at least be closer. Some legislators may be reluctant to break precedent, while others may be satisfied with changes made in the Education Department since last spring.

A new secretary of education has taken over, and the executive branch announced last month high school juniors will no longer be required to take the Smarter Balanced test.

Rep. Kowalko believes the vote “ought to be unanimous,” but he believes the chances of passage are much less certain.

Gov. Markell declined to predict the likelihood the veto override is successful Monday.

The Delaware Parent Teacher Association will host a rally at 1 p.m. today at Legislative Hall in support of the override.

An online petition backing the bill had garnered 525 signatures as of 5 p.m. Wednesday.