Stewart, who had run for governor in Trump's anti-establishment mold and derided Gillespie as "Establishment Ed," withheld his support after narrowly losing the June 13 primary.
At the time, Stewart declared that he would not back Gillespie unless the nominee took a harder line on protecting Confederate statues and against illegal immigration. "There is one word you will never hear from me, and that's unity," Stewart said in June.
But Gillespie, whose internal polls show him in a neck-and-neck race with Democrat Ralph Northam, needs voters who backed both Stewart and Trump to turn out for him.
Virginia is holding the only competitive statewide election in the nation on Nov. 7, an election that both major parties see as a test of electoral politics in the Trump era and a hint of what may come in the 2018 midterms.
Last month, Gillespie made an overture to that populist wing of the GOP, rolling out a series of hard-line ads against illegal immigration. And in recent days, he aired ads in which he defends Confederate statues.
Stewart confirmed his willingness to endorse and stump for Gillespie in an interview with The Washington Post on Tuesday, praising the candidate for "moving further to the right." He declined to comment on any behind-the-scenes communication with Gillespie, Bannon or Bossie.
"If it takes an endorsement to help defeat the radical, antifa left from taking over control of our state, I will do it," Stewart said, using a word used to describe left-wing, anti-fascist activists. "But whether that's an endorsement or bringing some energy to the base to show up and vote, whatever it takes, I will do whatever I'm asked to do."
Bannon and Bossie encouraged Stewart, who wants to challenge Sen. Tim Kaine (D-Va.) next year, to patch things up with Gillespie at a private meeting on Capitol Hill, according to the four people familiar with the matter, who spoke on the condition of anonymity because of the sensitivity of the negotiations between Gillespie and Stewart. Bossie, who lives in Maryland, is a member of the Republican National Committee.
Stewart broke the ice recently with a text message to Gillespie, but there was no response until after Stewart ran into Gillespie's wife, Cathy, at the Family Foundation's annual gala last weekend in Richmond, two of the people said.
Cathy Gillespie apologized to Stewart that her husband had not yet responded to his text. Soon afterward, Gillespie himself replied and the two have since texted back and forth.
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While it's not unusual for primary foes to bury the hatchet once a nomination is won, Stewart and Gillespie had a particularly bitter fight. Even after the primary, Stewart continued to criticize Gillespie in highly personal terms, at one point calling him boring and publicly advising him to "lose the dorky Mister Rogers sweater" that Gillespie favors in TV ads.
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Stewart, the chair of the Prince William Board of Supervisors, said he would not consider helping Gillespie unless he focused on issues Stewart cared most about, including illegal immigration and protection of Confederate monuments.
Gillespie, a former White House counselor to President George W. Bush and chair of the RNC, helped build the traditional Republican Party of the last 30 years. His critics say he embodies the Republican establishment that Trump and Stewart railed against.
Gillespie has struggled to energize Trump supporters without alienating moderates and independents that he needs to overcome a Democratic base that is united behind Northam. Gillespie's ambivalence toward Trump was on display last week, when the president tweeted his endorsement of the Republican but Gillespie did not mention it until the next day when asked by reporters.
Trump roils Virginia governor’s race with tweeted endorsement
Each of the players in the negotiations have something to gain.
For Stewart, any work he does to help Gillespie over the finish line on Nov. 7 could earn him goodwill in the state GOP in his primary fight next year.
Bossie and Bannon could notch a win for their wing of the party — and thwart the possible political takeaway that Trump's unpopularity sank Gillespie in this year's only competitive governor's race, and threatens to do the same for Republicans running in next year's congressional midterms.
Gillespie spokesman David Abrams did not respond directly to questions about whether a Stewart endorsement was in the works.
"I'm sure Ed's texted with Corey," Abrams said in a two-sentence statement that also referred to Gillespie's other GOP primary opponent. "Ed appreciates all that Corey Stewart and Frank Wagner have done to bring their party together in this critically important election."
Bossie declined to comment and Bannon was unavailable for comment.
Partnering with Stewart, however, raises the risk that Gillespie will energize Democrats and turn off moderates who were repulsed by Stewart's antics, which included rallies in front of the Confederate flag and raffling off a semiautomatic rifle to donors. Four of the five Republicans who serve with Stewart on the Prince William Board of County Supervisors all endorsed Gillespie during the primary because they objected to what they considered Stewart's divisive rhetoric.
Do Corey Stewarts antics help Gillespie or hurt the GOP brand?
Stewart said there are no complicated machinations behind his willingness to help Gillespie.
"There's no, quote, deal," Stewart said. "There's nothing there. I want to do whatever I can to help defeat the radical antifa-led Democratic Party."
He also said his role for Gillespie is yet to be determined.
"We haven't had a chance to sit down together and figure that out," he said. "I think these formal endorsements, the value of them is completely overblown. The fact that Ed is moving further to the right will help his campaign in terms of energizing the base."
Fenit Nirappil contributed to this report.