Hillary Clinton's campaign said that retired Republican senator John Warner of Virginia will endorse Clinton, marking the first time he has endorsed a Democratic presidential candidate and adding to the list of high-powered GOP figures stepping away from Donald Trump. (Video: WUSA9 / Photo: Kate Patterson/For The Washington Post)

Democratic presidential nominee Hillary Clinton will score another high-powered Republican endorsement on Wednesday, according to a campaign aide: retired senator John Warner of Virginia, a popular GOP maverick with renowned military credentials.

Warner’s decision not to support his party’s nominee, Donald Trump, is intended to send a signal in the five-term senator’s battleground home state and beyond that mainstream, security-minded Republicans should side with Clinton.

Virginia is an important, military-rich state that both candidates see as essential to winning the White House as the race tightens nationally. Clinton is making a pitch across the country that she is the more seasoned and responsible candidate on military and national security issues.

Perhaps best known by some for marrying actress Elizabeth Taylor, Warner, 89, is also known for bucking his party. A World War II veteran, former U.S. Navy secretary and former chairman of the Senate Armed Services Committee, Warner famously opposed the Supreme Court nomination of Robert Bork, as well as the 1994 Senate candidacy of Oliver North of Iran-contra notoriety. He endorsed Democrat Mark R. Warner over Republican Jim Gilmore to fill his own seat in the U.S. Senate.

John Warner’s ability to withstand the Republican criticism he endured for those decisions stemmed largely from the gravitas he had built over a lengthy Senate career in which he mastered national security issues and diligently delivered for the state’s military bases and defense contractors.

The list of Republicans who reject their presidential nominee, Donald Trump, and support his Democratic opponent keeps growing. (The Washington Post)

He has never before endorsed a Democrat for president.

“For 30 years, Virginians trusted John Warner in the Senate, and for good reason: He has dedicated his life to defending our country, from serving in the Navy in World War II to chairing the Senate Armed Services Committee, where I had the honor of working with him to support our men and women in uniform and their families,” Clinton wrote in a statement provided to The Washington Post. “I am proud to have John’s support, and to know that someone with his decades of experience would trust me with the weighty responsibility of being Commander in Chief.”

Warner is planning to make his endorsement official at an event in Alexandria with Sen. Tim Kaine (Va.), Clinton’s running mate, the Clinton aide said. The aide spoke on the condition of anonymity to preview the endorsement before it was announced.

Wednesday’s event, in a region where turnout is crucial to the Democratic ticket’s prospects in November, is expected to focus heavily on military issues. Kaine is a member of the Armed Services and Foreign Relations committees, and one of his sons is an active-duty Marine deployed overseas.

“The event will highlight how Hillary Clinton is by far the strongest and most experienced candidate on a full range of national security issues, and that she has the knowledge, steadiness and temperament to be commander in chief,” the Clinton aide said.

Despite a show of force of military leaders backing Clinton’s candidacy, Trump continues to claim more endorsements and to outperform her in polls of military veterans.

Warner retired in 2009, leaving office with approval ratings that any politician would covet. A Washington Post poll conducted a little more than a year before his retirement found that 72 percent of likely voters approved of the job he was doing.

Since becoming the nominee, Trump has struggled to corral the support of mainstream, establishment Republicans — notably in Virginia. But Clinton also has struggled to earn the trust of a majority of voters, particularly those associated with the military.

Clinton was leading Trump among likely Virginia voters 48 percent to 38 percent in a head-to-head matchup in a survey released Monday by the Wason Center for Public Policy at Christopher Newport University.

With three third-party candidates in the mix, Clinton has 39 percent to Trump’s 33 percent. Libertarian candidate Gary Johnson attracts 15 percent, while Green Party nominee Jill Stein and independent Evan McMullin each have 3 percent, according to the poll.

In military-heavy Virginia, more voters think Clinton would make a better commander in chief than Trump, 50 percent to 40 percent, with female voters saying so by a 2-to-1 margin. But military voters prefer Trump in that role 48 percent to 39 percent.

Surveying voters on character issues, the CNU poll found that 54 percent said Clinton cannot be trusted with classified information, while 53 percent said Trump is a racist.

Warner will become perhaps the most high-profile former GOP elected official whom Clinton has trotted out in recent weeks to try to make the case that she is the superior candidate on foreign affairs, in particular.

Other names well known in the foreign affairs establishment include John Negroponte, who held high-level positions in the administrations of Presidents Ronald Reagan and George W. Bush; Brent Scowcroft, a national security adviser to Presidents Gerald Ford and George H.W. Bush; and Richard Armitage, a deputy secretary of state in the George W. Bush administration.

Warner’s pick could also bolster the standing of the Democratic ticket in Virginia at a time when Republicans are trying to keep the state competitive. Not long ago, Republicans held the edge in presidential contests. A little more than a decade ago, President George W. Bush won Virginia with ease for a second time — an eight-point victory that was never in doubt.

Over the subsequent eight years, nearly the opposite happened: Barack Obama won the state twice, becoming the first Democrat to win Virginia since Lyndon B. Johnson in 1964.

With Kaine on the ticket this year, Clinton has maintained a lead that suggests a long-term realignment. As Trump has risen in the polls nationally, the race in Virginia has grown somewhat tighter, but the Democrats still enjoy a nearly 7 percentage point lead, according to the RealClearPolitics average of recent polls.

Kaine, a former Virginia governor and Richmond mayor, has been deployed far more often by the Clinton team to other battleground states around the country, including Florida, Ohio and Pennsylvania — a sign that Virginia is not the campaign’s steepest challenge.

About two weeks ago, Kaine held his first formal campaign events in Virginia — one in Norfolk and another just outside Richmond — since being named to Clinton’s ticket in July.

Other former elected Republican officeholders who have endorsed Clinton include: Michael Bloomberg, a mayor of New York; Arne Carlson, a governor of Minnesota; David Durenberger, a senator from Minnesota; Constance A. Morella, a congresswoman from Maryland; Larry Pressler, a senator and congressman from South Dakota; Christopher Shays, a congressman from Connecticut; and John J.H. Schwarz, a congressman from Michigan.

Clinton has also been endorsed by Richard L. Hanna, a current Republican congressman from New York.