PALO ALTO, Calif. – Lanhee Chen, one of the Republican Party’s most sought-after policy experts, said Monday he has signed on to Marco Rubio’s presidential campaign and will counsel the Florida senator on issues foreign and domestic.

As policy director on Mitt Romney’s 2012 presidential campaign, Chen was the GOP’s nominee’s chief policy adviser and a senior strategist. Since then, Chen has informally advised many Republican presidential hopefuls and other politicians from his perch here at Stanford University’s Hoover Institution.

Chen, who first announced his role on Hugh Hewitt’s syndicated radio show on Monday, said in an interview with The Washington Post that he has decided to work exclusively with Rubio because of what he considers the senator’s unique combination of policy depth and political potential.

"I went into it not sure what to expect, but I have been incredibly impressed with his grasp and command not just on domestic policy but also on foreign policy,” Chen said. “He is so well versed it's staggering. His ability to go from foreign policy to healthcare to taxes to social policy -- he's just so fluid, and that to me was super impressive. He represents the future of conservative thinking."

Chen’s role as a senior policy adviser with Rubio will be more informal than the full-time staff position he held on the Romney campaign. Chen, who is joining the Rubio team in a volunteer capacity, will continue to live in the San Francisco Bay Area, where he is a Hoover research fellow and teaches law and public policy classes at Stanford.

A close Romney confidant who in January helped advise the former nominee during his exploration of a third presidential run, Chen becomes the latest Romney alumnus to join the Rubio campaign.

Rubio’s campaign manager, Terry Sullivan, worked on Romney’s 2008 campaign, while his deputy campaign manager, Rich Beeson, was Romney’s national political director in 2012. Jim Merrill, a longtime New Hampshire strategist who ran Romney’s campaigns in the state, is playing the same role now for Rubio. And Poolhouse, a digital ad agency founded by Romney veterans Will Ritter and Tim O’Toole, handles Rubio’s media.

Chen said that he concluded, after months of discussions with several candidates, that Rubio was the Republican best positioned to win back the White House.

"He has a really unique combination of policy expertise first and foremost, political skill and communications ability,” Chen said. “You combine that with a really compelling life story and you have a candidate who will be successful not just in a primary but against whoever the Democrats nominate."