"I have decided to endorse Governor John Kasich for President because he is a leader who has a proven record of delivering results. John turned Ohio around at a tough time and I believe he can do the same for our country," Portman said in a statement released by the Kasich campaign.
Soon after the announcement, Portman tweeted his support.
".@JohnKasich has done a great job leading Ohio's comeback & I believe he is needed now to do the same for our country! -RP" he said.
The Ohio senator and Kasich have known each other for more than 20 years, having served in Congress together. Portman is also seeking re-election this year against former Ohio Gov. Ted Strickland in a race that is shaping up to be one of the most competitive Senate contests
"Rob is a tremendous asset for Ohio in the Senate and has played an important role in the recovery of jobs for our state," Kasich said in the statement. "I look forward to him returning to the Senate and working with him as President to advance the strategies that keep our economy strong and our country safe."
Portman, who will be the the national and Ohio co-chair for Kasich's campaign, served in Bush's second term as U.S. Trade Representative and director of the Office of Management and Budget.
Kasich is currently enmeshed in a battle with Bush's brother, Jeb Bush, to occupy the establishment lane in the Republican presidential primary, and a Kasich adviser told CNN that the campaign sees Portman's endorsement as a blow to Jeb Bush.
The adviser added that the campaign is working on getting Portman to appear at a campaign event with the Ohio governor.
A CNN/ORC nationwide poll
released late last month found both Kasich and Bush languishing toward the bottom of the GOP field in the low single digits.
But Kasich struck an upbeat tone Saturday morning in an interview with CNN's Michael Smerconish.
"We have to be a good story coming out of New Hampshire and we will be," Kasich said. "I'm like the little engine that can. People say, 'How is he going to do?' And you can see I'm rising in the polls. And that's what really matters. And we have the best ground game in New Hampshire with positive commercials on the air right now. And I'm predicting a very significant surge forward."