Six weeks ahead of the Republican primary, the woman seeking to oust Virginia House Speaker William J. Howell has suffered a setback: Her campaign manager quit.

The high-profile defection from Susan Stimpson’s campaign comes as the two vie for conservatives in the Fredericksburg-area district, about 50 miles south of Washington.

Stimpson, a one-time protege of Howell’s, has accused the longtime incumbent of supporting bipartisan tax increases that gave the state’s Democrats a leg up to higher office.

Since last summer’s surprise defeat of former House majority leader Eric Cantor, Howell has taken Stimpson’s challenge seriously in hopes of avoiding the same fate, and he touted his opposition to Medicaid expansion as evidence of his conservative credibility.

Rollin L. Reisinger declined to comment on why he left the campaign. He was brought on board last year by Tim Edson, general consultant to the campaign, who also ran Stimpson’s failed bid for lieutenant governor in 2013. Edson declined to comment.

There has been no public polling for the race. A few thousand people are expected to vote.

Experts say that as one of the state’s most powerful Republicans, Howell, 71, has a built-in advantage in name recognition and fundraising strength, giving Stimpson an uphill battle to the nomination.

Stephen J. Farnsworth, a political science professor at the University of Mary Washington in Fredericksburg, said the internal shake-up could stall Stimpson’s momentum.

“For Stimpson to beat an established figure like Bill Howell, that campaign would need to fire on all cylinders, but this latest development suggests the Stimpson campaign is actually sputtering,” he said.

John Fredericks, who first reported Reisinger’s resignation on his radio show on 1650 AM, praised Stimpson as intelligent and eloquent and said she makes a positive impression on the trail. But from the start of her campaign, he said, she “looked to me like a candidacy in search of a purpose.”

Stimpson, hoping to harness the tea-party-fueled discontent that sent Rep. Dave Brat (R-Va.) to Congress, blamed Howell for letting through the General Assembly legislation that would have increased taxes and supporting a $1.2 billion-a-year transportation funding overhaul in 2013.

Her supporters include Grover Norquist of Americans for Tax Reform, who counts her among the signers of his anti-tax pledge.

“He’s been a placeholder and friend of tax-hiking special interests in Richmond,” ATR spokesman Paul Blair said of Howell. “Why should voters accept a placeholder instead of a conservative leader willing to be bold? If Bill Howell wins in June, taxpayers will be the real losers.”

Howell has called the transportation bill vital for residents’ safety as well as economic development. And he has sought conservative support for his role in another partisan fight: Medicaid expansion. Last year, Howell used his supermajority in the House to block Democratic Gov. Terry McAuliffe’s effort to provide federally funded health care to 400,000 uninsured Virginians.

“As we have been all along, we’re focused on running our campaign and sharing the Speaker’s record of proven, conservative leadership with the voters of the 28th District,” spokesman Matt Moran said.

Howell’s fundraising amassed more than $165,738 in the first three months of the year and wrapped up March with a balance of more than $179,642, according to the Virginia Public Access Project.

His top donors include the Republican GOPAC, Verizon and the Virginia Cable Telecommunications Association. He also controls Dominion Leadership Trust, a political action committee with nearly $900,000 to spend on a few competitive races this year.

Stimpson raised $46,134 during the same time period and has a balance of $31,259.

Some have speculated that Stimpson would have had an easier path to the nomination had she simply waited for Howell to retire.

Farnsworth said, “It’s pretty clear getting into the race energized the speaker’s campaign efforts in a way we haven’t seen in this district in a long time.”

The primary is June 9.