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 PortmanRobert (Rob) Jones PortmanTax rules will be subject to more OMB review under new memo Ending sex trafficking tomorrow requires preventing child abuse today Doctors bristle at push for opioid prescription limits MORE (R-Ohio), John ThuneJohn Randolph ThuneHouse, Senate GOP compete for cash Overnight Tech: Alleged robocall kingpin testifies before Congress | What lawmakers learned | Push for new robocall rules | Facebook changes privacy settings ahead of new data law | Time Warner CEO defends AT&T merger at trial Senators grill alleged robocall kingpin MORE (R-S.D.), and Marco RubioMarco Antonio RubioFormer Florida congressmen mull bipartisan gubernatorial run: report Winners and losers from Jim Bridenstine’s confirmation as NASA administrator GOP Senate candidates trade barbs in brutal Indiana primary MORE (R-Fla.) have made the rumored shortlist along with Rep. Paul RyanPaul Davis RyanWatchdog group sues for donor list from Ryan-aligned nonprofit Terminating Budget Committees not as absurd as it sounds The writing is on the wall for bump stocks and Congress should finalize it 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.