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Tester announced his support in a message on his Facebook on Tuesday. 

"Montanans believe in the right to make a good life for their families. How they define a family should be their business and their business alone," Tester wrote. "I’m proud to support marriage equality because no one should be able to tell a Montanan or any American who they can love and who they can marry."

Tester's statement follows similar announcements from a number of senators in recent days. Democratic Sens. Mark BegichMark Peter BegichPerez creates advisory team for DNC transition The future of the Arctic 2016’s battle for the Senate: A shifting map MORE (Alaska), Claire McCaskillClaire Conner McCaskillOvernight Cybersecurity: Homeland Security official says Russia likely targeted more than 21 states | Senate approves Trump's NSA chief | Lawmakers unveil bipartisan internet privacy bill Senators debate new business deduction, debt in tax law hearing Russia likely targeted more than 21 states before 2016 election: official MORE (Mo.), Jay RockefellerJohn (Jay) Davison RockefellerSenate GOP rejects Trump’s call to go big on gun legislation Overnight Tech: Trump nominates Dem to FCC | Facebook pulls suspected baseball gunman's pages | Uber board member resigns after sexist comment Trump nominates former FCC Dem for another term MORE (W.Va.), and Mark WarnerMark Robert WarnerOvernight Finance: Stocks fall hard | Trump sending delegation to China for trade talks | SEC fines Yahoo M over breach | Dodd-Frank rollback dominates banking conference Trump’s CIA pick facing brutal confirmation fight Rand's reversal advances Pompeo MORE (Va.) have all come out in support of same-sex marriage.

Those announcements follow two other similar statements by Sen. Rob PortmanRobert (Rob) Jones PortmanSenators debate new business deduction, debt in tax law hearing Tax rules will be subject to more OMB review under new memo Ending sex trafficking tomorrow requires preventing child abuse today MORE (R-Ohio) and former Secretary of State Hillary ClintonHillary Diane Rodham ClintonDems flip New York state seat that Republicans have held for nearly four decades Dems win majority in New York Senate, but won't control it Chelsea Clinton hits back at NYT reporter over details in new book MORE who, about a week earlier, both said they support same-sex marriage.

Tester's announcement on Tuesday came the same day that the Supreme Court began hearing arguments on a case on same-sex marriage.