Buck, who is expected to be a spokesman for the VP nominee when a selection is made sometime this month, will be working with Kevin Sheridan, a former Republican National Committee spokesman who will head up the communications shop in the vice presidential pick's office, and Romney national press secretary Andrea Saul.

He took leave from the speaker's office and started the job this week in Boston.

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The move by Buck, from Capitol Hill to Mitt Romney's operation, could raise speculation that the vice presidential pick could come from Congress.

To date, Sens. Rob PortmanRob PortmanGOP senators bristle at Trump's Medicaid cuts Overnight Energy: Trump budget takes flak over oil provisions GOP senators knock Trump's budget proposal MORE (R-Ohio), John ThuneJohn ThuneGOP frustrated by slow pace of Trump staffing Senate takes lead on Trump’s infrastructure proposal Senate feels pressure for summer healthcare vote MORE (R-S.D.), and Marco RubioMarco RubioMcConnell on Trump: 'We could do with a little less drama' Taking the easy layup: Why brain cancer patients depend on it Rubio: 'All options should be on table' if Flynn refuses new subpoenas MORE (R-Fla.) have made the rumored shortlist along with Rep. Paul RyanPaul RyanBorder-adjustment tax proposal at death’s door Overnight Healthcare: CBO fallout | GOP senators distance themselves from House bill | Trump budget chief blasts score | Schumer says House bill belongs 'in the trash' Five takeaways from the CBO healthcare score MORE (R-Wis.).

"Brendan will be a great addition to the campaign. We’re looking forward to him applying his quick wit and deep understanding of President Obama’s record of broken promises to our effort. We’re lucky to have him," said Sheridan.

— This story was updated at 12:03 p.m.