(CNN) -- The Illinois Senate will vote Thursday -- Valentine's Day -- on whether to legalize same-sex marriage.
Because Democrats have supermajority control of the General Assembly, the measure is expected to be approved. After the Senate vote, the measure would be considered by the House.
Gov. Pat Quinn, a Democrat, has indicated he would sign the bill.
If it is approved, Illinois would be the 10th state, plus the District of Columbia, to legalize same-sex marriage, according to Lambda Legal, a gay rights organization.
Three other states are considering similar legalization, said Camilla Taylor, marriage project director for Lambda Legal. A bill has passed the Rhode Island House and has been sent to the Senate. A proposal has been introduced in the Hawaii legislature and another is expected in Delaware, Taylor said.
"All eyes are on us," said Taylor, who is based in Illinois. "It's looking great. We're very excited about Illinois."
On February 6, Quinn said that "marriage equality is a matter of fairness and equal rights under law."
"We took the first step towards marriage equality two years ago when I signed civil unions into law. Since that day, thousands of committed couples in 92 counties across our state have entered into civil unions," he said. "Now is the time for the next step in providing equal rights to all people in Illinois."
Five states, including Illinois, have civil union laws, according to Lambda Legal and the National Conference of State Legislatures. The others are Hawaii, Delaware, New Jersey and Rhode Island.
The groups list four states as having some type of domestic-partnership recognition: California, Oregon, Nevada and Wisconsin. Lambda Legal lists Colorado as recognizing designated beneficiary agreements.
Twenty-nine states have constitutional provisions banning same-sex marriage, the NCSL says.
Polls show that legalizing same-sex marriage is gaining support across the country, although it remains less popular in some Midwestern and Southern states than in the rest of the country.
Even Illinois Republican Party Chairman Pat Brady said in January that he supported same-sex marriage and urged legislators to vote for it. The move prompted criticism from conservative Republicans, including state Sen. Jim Oberweis.
But Oberweis seemed resigned to Republicans lacking the votes to stop the bill.
"I hope that we resolve this issue sooner rather than later because the state of Illinois has some tremendous financial problems to deal with, which in my view is where we should be concentrating our focus and our time," he said.