Sen. Claire McCaskillClaire Conner McCaskillDemocrats turn on Al Franken Trump rips Dems a day ahead of key White House meeting The Hill's 12:30 Report MORE (D-Mo.) announced her support for same-sex marriage Sunday night, becoming one of the first Democratic senators from a traditionally Republican state to do so.

McCaskill's statement, posted to her Tumblr page, precedes oral arguments at the Supreme Court on Tuesday and Wednesday on a pair of cases examining states' and the federal government's ability to define marriage as between a man and a woman.

"I have come to the conclusion that our government should not limit the right to marry based on who you love," McCaskill wrote. "While churches should never be required to conduct marriages outside of their religious beliefs, neither should the government tell people who they have a right to marry."

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McCaskill, who was reelected to a second term in the Senate last year, joins a growing number of lawmakers advocating for same-sex marriage. Earlier this month, Republican Sen. Rob PortmanRobert (Rob) Jones PortmanMcConnell names Senate GOP tax conferees Overnight Finance: House approves motion to go to tax conference — with drama | GOP leaders to consider Dec. 30 spending bill | Justices skeptical of ban on sports betting | Mulvaney won't fire official who sued him How four GOP senators guided a tax-bill victory behind the scenes MORE (Ohio) switched his position in favor of same-sex marriage, citing his college-aged son who is gay.

Former Secretary of State Hillary ClintonHillary Diane Rodham ClintonGrassley blasts Democrats over unwillingness to probe Clinton GOP lawmakers cite new allegations of political bias in FBI Top intel Dem: Trump Jr. refused to answer questions about Trump Tower discussions with father MORE made a splashy endorsement of gay marriage last week, reflecting the broader shift the Democratic Party has made on the issue in recent years. 

In her post, McCaskill says she was influenced by friends and colleagues in committed, same-sex relationships.

"My views on this subject have changed over time, but as many of my gay and lesbian friends, colleagues and staff embrace long term committed relationships, I find myself unable to look them in the eye without honestly confronting this uncomfortable inequality," McCaskill wrote. "Supporting marriage equality for gay and lesbian couples is simply the right thing to do for our country, a country founded on the principals of liberty and equality."

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She also acknowledges that attitudes on gay marriage seem to be shifting. A Washington Post / ABC News poll released last week showed that 58 percent of Americans backed gay marriage, an all-time high.

"Good people disagree with me," McCaskill said. "On the other hand, my children have a hard time understanding why this is even controversial. I think history will agree with my children."