(CNN) -- When Rob Portman signed on to the Mitt Romney campaign in January 2012, he was seen as a key supporter -- a rising star in the Republican Party, a swing state campaigner and a staunch conservative who could protect Romney on the right.
For the next 11 months, Portman zipped around the country, pitching the candidate he said was "exactly what our nation needs."
The Ohio senator's hard-nosed approach landed him on Romney's vice presidential short list. He was even vetted for the job.
The Romney campaign was undoubtedly the height of Portman's political career.
After he failed to deliver his state and Romney lost the election, Portman stepped out of the limelight and remained relatively quiet on Capitol Hill as President Barack Obama settled in for a second term.
That's when Portman, a former White House budget director, was thrust into the divisive national debate when he publicly acknowledged that he had changed his mind on same-sex marriage.
"I'm announcing today a change of heart on an issue that a lot of people feel strongly about that has to do with gay couples' opportunity to marry," Portman told CNN Chief Congressional Correspondent Dana Bash two years after learning his son, Will, was gay.
"I've come to the conclusion that for me, personally, I think this is something that we should allow people to do, to get married, and to have the joy and stability of marriage that I've had for over 26 years. That I want all of my children to have, including our son, who is gay," Portman said.
Portman is an unexpected advocate for same-sex marriage -- and he admitted as much in the interview.
"What happened to me is really personal," Portman said. "I mean, I hadn't thought a lot about this issue. Again, my focus has been on other issues over my public policy career."
Even though he is seen as a social conservative, someone whose social policy is deeply rooted in his Christian faith, Portman is noticeably out of his comfort zone in this discussion. It is budget and tax issues, not marriage and sexuality, that he is better known for.
Portman was first elected to the Senate in 2010 after receiving little challenge from Democrat Lee Fisher for a seat vacated by Republican George Voinovich.
While there, he has pushed fiscal issues, including a balanced budget amendment.
This focus tracks with Portman's past.
His first foray into elected office was when he won a House seat in 1993, representing Ohio's 2nd Congressional District.
In that role, he began to hone his expertise on budget and fiscal issues. During his 12 years in the House, Portman sat on the Ways and Means and Budget committees -- two powerful budget and tax-writing panels -- and co-chaired the National Commission on Restructuring the Internal Revenue Service.
His expertise was noticed by the George W. Bush White House.
Bush made Portman the U.S. trade representative in 2005 and he became the Republican president's budget director the next year.
The job required Portman to dive into the esoteric world of federal spending. On his website, Portman describes himself as "a deficit hawk" that proposed "a balanced budget," fought "irresponsible earmarks" and advocated for more transparency.
Not everyone agrees with that assessment, however.
Portman was only at the budget office for a year where he worked with the White House on its fiscal 2008 budget -- a document that led to a deficit of $459 billion.
When Portman was rumored as a possible vice presidential candidate for Romney, nearly all Democratic operatives attacked him for his time as Bush's top budget deputy.
Many said he portrayed himself as a deficit hawk, but didn't live up to those ideals when he worked for the White House.
Greg Schultz, the Ohio state director for Obama's 2012 campaign, stressed these critiques in a blog post for the campaign.
"As one of the architects of the top-down Bush budget, Portman practically invented the policies that punished middle-class families while exploding the deficit, and crashing our economy," Schultz wrote.
Portman, in the middle of the presidential campaign, even tried to distance himself from the Bush administration.
"I was frustrated when I was there about some spending issues — specifically, as you know, I wanted to offer a balanced budget over five years, and a lot of people didn't," Portman said in an interview with the Hill Magazine. "I prevailed. The president sent his budget — not my budget, his budget — a five-year balanced budget. But it was a fight, internally."