Ohio Senator Rob Portman on Tuesday said that he has decided not to run for president in 2016.

“While I appreciate the encouragement I have received from many to run for president, my focus will remain in Ohio and running for re-election to the Senate in 2016,” Portman said in a statement.

“That's where I believe I can play the most constructive role,” he added. “I don't think I can run for president and be an effective senator at the same time.”

The 58-year-old Portman last year became the first sitting GOP senator to endorse marriage equality.

In an op-ed and later in interviews with CNN and the AP, Portman, a co-sponsor of the Defense of Marriage Act (DOMA), said that he had dropped his opposition to marriage equality after his son, Will Portman, told him and his wife Jane that he's gay.

Last month, the National Organization for Marriage (NOM) pledged to target Portman for defeat in 2016.

“Rob Portman can forget about getting elected President of the United States,” NOM President Brian Brown said. “If he runs we will make sure that GOP primary voters are aware of his desire to redefine marriage and his willingness to see federal judges set aside the votes of 50 million Americans who enacted marriage amendments across the country because his son is gay.”

“We intend to oppose Sen. Portman for reelection, and if he survives a primary challenge we will urge Republicans and Independents to refuse to vote for him in the General Election,” he added.