Former House speaker Newt Gingrich is on Republican presidential candidate Donald Trump's short list of potential vice presidential candidates. Here's what you need to know about him. (Sarah Parnass,Danielle Kunitz,Osman Malik/The Washington Post)

Former House speaker Newt Gingrich is on Republican presidential candidate Donald Trump's short list of potential vice presidential candidates. Here's what you need to know about him. (Sarah Parnass,Danielle Kunitz,Osman Malik/The Washington Post)

Donald Trump’s campaign has begun formally vetting possible running mates, with former House speaker Newt Gingrich emerging as the leading candidate, followed by New Jersey Gov. Chris Christie. But there are more than a half dozen others being discussed as possibilities, according to several people with knowledge of the process.

Given Trump’s unpredictability, campaign associates caution that the presumptive Republican nominee could still shake up his shortlist. But with little more than two weeks before the start of the Republican National Convention, Gingrich and Christie have been asked to submit documents and are being cast as favorites for the post inside the campaign. Gingrich in particular is the beneficiary of a drumbeat of support from Trump confidants such as Ben Carson.

A number of senators — including Jeff Sessions (Ala.) and Bob Corker (Tenn.) — are also being reviewed as viable picks, although the extent to which they are being vetted is unclear. A longer shot on Trump’s radar is Indiana Gov. Mike Pence, a heavyweight on the right who could bolster Trump’s tepid support among some conservative activists.

But Pence is immersed in his reelection race and Trump is said to want a more electric politician at his side rather than a low-profile figure. Most of his primary rivals are reluctant to sign on, and tensions with Ohio Gov. John Kasich and Sen. Ted Cruz (Tex.) remain raw.

Details of the running-mate search were provided by five people with knowledge of the process who requested anonymity to discuss private conversations with campaign officials.

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Gingrich, who said on “Fox News Sunday” over the weekend that “nobody has called me” from the Trump campaign about the possibility of being vice president, declined to comment. Campaign spokeswoman Hope Hicks also declined to comment. Christie’s office did not respond to an inquiry.

The contenders under the most serious consideration, such as Gingrich and Christie, have been asked by attorney Arthur B. Culvahouse Jr. to answer more than 100 questions and to provide reams of personal and professional files that include tax records and any articles or books they have published.

Culvahouse, a former White House counsel who is managing the vetting for Trump, was the lawyer who vetted then-Alaska Gov. Sarah Palin for the GOP vice-presidential nomination during the 2008 campaign.

The narrowing list of running-mate possibilities comes at the end of a turbulent period for Trump, who has struggled to raise money since clinching the GOP nomination and has stumbled through a series of self-inflicted controversies, including a racially charged attack on a sitting federal judge and a continuing outcry over his rhetoric against Muslims and other minorities.

The presumptive Republican nominee continues to indicate that he will probably choose someone who could balance his brash populist persona with a political profile that includes deep experience in Washington or ties to the party establishment, the people familiar with the search said.

The timing of Trump’s announcement was for months expected to happen close to the convention. But campaign aides are now discussing moving it up, perhaps to later next week so the ticket can generate headlines and coverage — and win over party leaders — ahead of the party gathering in Cleveland.

Donald Trump and former House speaker Newt Gingrich talk to the media after a meeting in New York in 2011. (Seth Wenig/AP)

With Gingrich, 73, or Christie, 53, the 70-year-old mogul would be joined by a well-connected Republican who shares his combative style and his ease at being a ubiquitous media presence. Both men have won Trump’s favor by actively supporting him — Gingrich primarily through television appearances and Christie through behind-the-scenes talks with party leaders and leading GOP donors.

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Their experience facing down and cutting deals with Democrats has also drawn the interest of Trump, who has acknowledged that he would be a novice at working directly with lawmakers.

Gingrich would bring with him a history of battling with presumptive Democratic nominee Hillary Clinton, going back to their public fights over health care and Bill Clinton’s agenda and ultimate impeachment in the 1990s during her husband’s presidency. For years, Gingrich was seen by Clinton allies not just as an opponent but a nemesis with a penchant for grandiose rhetoric and barbed attacks — traits that Trump is said to welcome.

Trump himself has heavily turned to elements of the 1990s in recent weeks on the campaign, revisiting past Clinton-related scandals and issues as he builds his case against the former secretary of state.

Sessions and Corker are among the other names mentioned by people who have spoken with Trump officials. Sessions, a conservative populist who was the first senator to endorse Trump last year, has seen many of his trusted aides take on high-ranking roles in the Trump campaign. Corker, the chairman of the Senate Foreign Relations Committee, has voiced support for some of Trump’s views.

Sens. Richard Burr (N.C.), Tom Cotton (Ark.) and John Thune (S.D.) have also been bandied about in Trump Tower as options. Sen. Joni Ernst (Iowa) and Oklahoma Gov. Mary Fallin, who previously served in the House, are two of the leading women in the mix.

But shortcomings for many of these candidates have made their chances seem less likely to Trump advisers.

Pence, Thune and Burr would bring heft and have held leadership positions, but they are focused on their reelection bids. Corker is well liked by campaign chairman Paul Manafort, but his recent public criticisms of Trump’s tone and statements have not been welcomed by the candidate.

Cruz is seen as someone Trump would like to bring into the fold because of his political capital with the conservative movement. But their bitter clashes during the primary have left a mark, and Cruz has so far declined to endorse Trump. That has not stopped members of Trump’s team from reaching out to members of Cruz’s circle and trying for a reconciliation.

Trump’s desire for a governing partner is not the only factor that has been mentioned in discussions among aides. Contenders’ rapport with the mogul and their ability to comfortably communicate and defend his nontraditional platform are also crucial, the people familiar with the process said.

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Less central have been the candidates’ home states or regional influence, given that Trump sees the campaign as a nationalized political war that is largely being fought on television.

Trump’s inclination toward naming a seasoned player has been encouraged by Manafort, the longtime GOP insider who has taken full control of the process following Trump’s firing of Corey Lewandowski, who had been campaign manager.

Yet even as Manafort steers the selection and as members of Trump’s orbit — especially his children and son-in-law Jared Kushner — informally weigh in, there is a collective understanding within the campaign that Trump’s voice is the only voice that matters. One person involved in the process suggested the ultimate decision will come down to a committee of one: Trump. “This is in his head,” the person said. “It’s up to him.”

Robert Jeffress, a Dallas pastor who has become close with Trump during the campaign, said in an interview that while he has not spoken to Trump about the vice-presidential slot, Trump has made clear that he “wants someone who can help get his legislative agenda through Congress.”

“I think that is how he is going,” Jeffress said. “He’d be coming in as an outsider, and that has fueled his popularity. But he is the first to admit that he doesn’t know all the ways of Washington. So to actually push what he wants through, he’s willing to reach out and get somebody to lend a hand.”