Senate Joint Resolution 39
, dubbed by its supporters as a "religious freedom" bill, proposes an amendment to the state's constitution over the "protection of certain religious organizations and individuals from being penalized by the state because of their sincere religious beliefs or practices concerning marriage between two persons of the same sex."
"This bill is a direct hit on those individuals who decide to love and be in love with the same sex, and that's not fair," said Sen. Jamilah Nasheed, one of eight Democratic state senators trying to stop the bill from passing.
The bill's sponsor told CNN it has a narrow focus.
"We spent a lot of time writing it to avoid the controversies we've seen in other states," Republican Sen. Bob Onder said.
He told his colleagues in the Senate that the bill "protects churches, pastors, religious organizations in a very well-defined class of individuals from being penalized, targeted, persecuted on the basis of their religious beliefs."
But opponents of the measure, including the American Civil Liberties Union and the PROMO advocacy group, have called it "anti-LGBT legislation." They compared it to Indiana's controversial religious freedom law
that lawmakers there passed last year.
"These dangerous bills and potential constitutional amendments only succeed in showing people Missouri is not a welcoming state," the organizations said in a letter to lawmakers last month
. "We should focus on keeping Missouri competitive, not keep people away."
From Senate floor to social media
The filibuster began Monday afternoon. By Wednesday morning, it had lasted more than 34 hours.
Democratic lawmakers said they had no plan to stop.
If the measure passes the state's Senate and House, Missouri voters will have the final say. To become part of the constitution, a majority of voters must approve it in a statewide vote.
As the filibuster stretched into its second day Tuesday, it drew attention on social media, where supporters -- and some critics -- weighed in using the hashtag #NotInMyState.
Missouri Gov. Jay Nixon, a Democrat, praised lawmakers for their efforts.
Wendy Davis, a Texas state senator who drew national attention when she filibustered a controversial abortion bill in her state, said she'd be happy to let Missouri lawmakers borrow her shoes as they stand up for their beliefs.
Critics accused Democrats of being selective about whose religious freedom matters.
And the Senate Majority Caucus fired back at Democrats with tweets of its own, saying Republican lawmakers were fighting for fairness.
The filibuster drew praise from Democrats on the campaign trail, with both former Secretary of State Hillary Clinton and Vermont Sen. Bernie Sanders tweeting their support.