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 debates going big on tax reform Who is Tim Ryan? A closer look at Pelosi’s challenger Battle for the Senate: Top of ticket dominates MORE (R-Ohio), John ThuneJohn ThuneWeek ahead: AT&T-Time Warner merger under scrutiny Fight breaks out at FCC over 'zero-rating' data plans A political temper tantrum at the FCC MORE (R-S.D.), and Marco RubioMarco RubioJuan Williams: McConnell won big by blocking Obama The ignored question: What does the future Republican Party look like? Graham to roll out extension of Obama immigration program MORE (R-Fla.) have made the rumored shortlist along with Rep. Paul RyanPaul RyanEx-comptroller: Trump will try to govern like Reagan Communities struggling with decline of coal can’t wait any longer on RECLAIM Act Juan Williams: McConnell won big by blocking Obama 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.