HB1240 passed the Senate in a 27-21 vote, with one excused. The measure will go back to the House since it was amended by the Senate before it is sent to Gov. Jay Inslee.
The Washington state House in March passed a measure on a largely party-line 55-42 vote.
If the bill is signed into law, Washington would join nine other states that have enacted laws generally banning the sale, manufacture and transfer of assault weapons, according to the Giffords Law Center to Prevent Gun Violence.
Attorney General Bob Ferguson praised the move saying that "The Senate put public safety above the interest of the gun lobby."
Democratic Gov. Jay Inslee watched the House vote in the chamber last month and called it very important.
"It is something that I’ve believed in since 1994 when I voted to make this federal law," said Inslee, who served in Congress before becoming governor.
He said the Legislature had broken "the NRA’s lock on Washington" and that "the vast majority of Washingtonians support this commonsense measure."
With gun deaths hitting the highest mark seen in decades in the U.S., Democrats in Washington state have flexed their majorities to push the assault-weapon ban while advancing other gun restrictions, including a 10-day waiting period for purchases and a bill that would hold gunmakers liable for negligent sales.
During the debate on the assault-weapons bill, Democrats spoke of mass shootings frequently carried out by men wielding assault-style weapons such as AR-15s that have killed people in schools, churches, nightclubs and grocery stores.
State Rep. Darya Farivar, D-Seattle, lamented a modern cycle of "deaths, sadness, thoughts and prayers, and then back to business." She said she has experienced a shooter lockdown, and she decried the fearful reality faced by schoolchildren.
Republicans argued the ban violates the federal and state constitutions and predicted it would be overturned. They also contend it would not stop criminals or prevent mass shootings while infringing on the rights of law-abiding people seeking to defend their families.
"Firearms are the great equalizer," said state Rep. Travis Couture, R-Allyn, touting their use to defend against "all sorts of predators."
The House bill lists more than 50 specific gun models that would be banned from future sales in the state, including military-style weapons such as AR-15s, AK-47s and M-16s. It also lists characteristics of prohibited firearms, such as semi-automatic rifles with a length of less than 30 inches, and those that have detachable magazines or fixed magazines with a capacity of ten rounds or more.
Lawmakers have advanced the firearm restrictions as part of a package announced in December by Inslee and Attorney General Bob Ferguson.
The state House also in March passed House Bill 1143, requiring a 10-day waiting period and safety training before all gun purchases. They had already been required for semi-automatic rifles.
The state Senate last week approved a bill that would give the Washington Attorney General’s office the authority to hold gun manufacturers or sellers liable for gun sales that result in weapons landing in the hands of criminals or mass shooters.
Each gun measures must still be considered and voted on in the coming weeks by the opposite chamber in the legislative session, set to continue into late April.
The Associated Press contributed to this report.