RICHMOND — Gov. Ralph Northam would boost K-12 spending by nearly $269 million, increasing teachers’ pay raises to 5 percent and plowing more money into school construction, per-pupil spending and programs for at-risk students under a budget plan partially unveiled Tuesday.

Northam (D) said the state had a “once-in-a-lifetime” chance to make those investments because of a windfall of up to $600 million from changes to federal tax law and a recent U.S. Supreme Court ruling that allows states to require online retailers to collect sales taxes.

To a lesser extent, higher-than-anticipated tax and lottery revenue would fund some of his plan, which also calls for boosting the amount in the state’s reserve fund.

“There is power in every child,” Northam, a pediatrician, said at a news conference at the state Capitol attended by first lady Pam Northam, a former science teacher. “I’ve been clear since my first day in office that delivering on the promise of a high-quality public education for each and every child in the commonwealth is one of our highest priorities and greatest responsibilities.”

The governor said the raises would help the state attract and retain teachers, whose average pay lags behind the national average by about $9,000 a year. The raises would build on a 3 percent hike already included in the two-year, $115 billion budget that took effect July 1, adding another 2 percent effective July 2019.

Jim Livingston, president of the Virginia Education Association, said the governor’s plan will help a state that started the school year with 1,000 teaching positions unfilled.

“That’s 1,000 classrooms across the commonwealth that don’t have a certified teacher,” he said. “Those students are suffering. They’re being left out because they don’t have a qualified individual standing at the front of their classroom.”

Also cheering the governor’s plan was Michael Cassidy, president of the Commonwealth Institute for Fiscal Analysis, a liberal-leaning think tank.

“We need to turn the corner on state support for public education,” Cassidy said. “All of the attention across the country, with these teacher walkouts and strikes, has shined a particularly powerful light on that issue of teacher pay.”

The plan got a more cautious reception from Republicans, who have narrow control over the state House and Senate. Their support will be needed to pass legislation on the Internet tax and to conform state tax code to the changes made at the federal level, which would result in a windfall because some deductions would be eliminated. Some legislators have ideas of their own for how to spend any extra money.

Northam has also proposed a plan for using half of the money from the federal tax changes for tax rebates for Virginians making less than $50,000 a year — a plan that has met with some GOP resistance.

“We appreciate the Governor’s proposal and will consider it when drafting our budget amendments,” said Del. S. Chris Jones (R-Suffolk), chairman of the House Appropriations Committee. “K-12 education has been a priority of House Republicans and we are glad to see the Governor building on the raises and over $1 billion in new funding for K-12 education provided in our budget last year. However, we have to remember this is just one piece of the broader package that we will consider as we put together our proposals.”

In June, the Supreme Court ruled that states can require online retailers to collect sales taxes. A bipartisan group of senators has proposed a bill that would devote $150 million to $200 million a year — half of the proceeds the from Internet sales taxes — to purchase up to $4 billion in construction bonds for schools around the state.

Sen. William Stanley (R-Franklin), one of the sponsors of that bill, praised Northam’s attention to school infrastructure needs. But he said the governor’s plan — which would make a one-time, $80 million deposit in a school construction fund — does not go far enough.

Stanley’s co-sponsors include Sens. Barbara Favola (D-Arlington), Dave Marsden (D-Fairfax) and Glen Sturtevant (R-Richmond). “This [$80 million plan] is not going to be able to address the statewide problem that we have,” he said, describing urban schools where ceiling tiles routinely fall and rural ones with just one electrical outlet per room.

Northam made reference to their plan Tuesday, questioning whether the sales tax would bring in as much as they were projecting.

Earlier this year, the General Assembly passed a two-year budget that Northam signed into law. The legislature and governor typically make adjustments to the spending plan during the course of those two years. Northam is scheduled to propose his amendments before a joint session of the House and Senate money committees next week. The governor’s announcement Tuesday was the first of several proposals that he is expected to roll out ahead of that.

Northam called for increasing spending on a program for at-risk students by $35 million. Named At-Risk Add-On, the program provides certain low-income schools with dropout-prevention services, after-school programs and specialized instruction. He would spend another $35 million to boost per pupil spending across the state. Local school systems will have flexibility to decide how to spend those funds.