Hillary Clinton’s campaign hopes that there are many more national-­security-minded Republicans and independents who would vote for her, even grudgingly, rather than see Donald Trump win the White House. (Melina Mara/The Washington Post)

Retired Army Col. Peter Mansoor plans to vote for Hillary Clinton for president this year, but not because the longtime Republican and former top aide to then-Gen. David Petraeus has had a political conversion. He just thinks Republican Donald Trump is too dangerous to be president.

“It will be the first Democratic presidential candidate I’ve voted for in my adult life,” said Mansoor, a professor of military history at Ohio State University.

Clinton’s campaign hopes that there are many more national-­security-minded Republicans and independents who would vote for her, even grudgingly, rather than see Trump win the White House. Those voters are an important part of the audience for her case that she is fit to be commander in chief and that Trump is not.

Clinton has begun making that argument more forcefully as her long primary battle grinds to a close. She will deliver what her campaign calls a major foreign policy address in California on Thursday, focused both on her ideas and leadership credentials and on what she will describe as the threat Trump poses to national security.

“Clinton will rebuke the fear, bigotry and misplaced defeatism that Trump has been selling to the American people,” an aide said. “She will make the affirmative case for the exceptional role America has played and must continue to play in order to keep our country safe and our economy growing.”

Democratic presidential candidate Hillary Clinton said she's "never heard such reckless, risky talk" about nuclear weapons from a future presidential nominee than from Donald Trump during a rally in Louisville, Ky. (Reuters)

The aide spoke on the condition of anonymity to outline the plan for the speech, which has not been previously reported.

The address will expand on themes Clinton sketched in a CNN interview in May, when she flatly said Trump, the presumptive Republican nominee, is not qualified to be president. She ticked through positions the businessman has taken during a campaign few thought he could win. Among them: an apparent willingness to back out of the NATO alliance; a suggestion that the U.S. defense burden would be lightened if Japan and other nations acquired nuclear weapons; and his pledge to bar foreign Muslims from entering the United States.

“I know how hard this job is, and I know that we need steadiness, as well as strength and smarts in it, and I have concluded that he is not qualified to be president of the United States,” Clinton said.

The speech Thursday in San Diego marks a turning point toward an argument that, by design, has not been as large a part of the primary campaign as Democrats expect it to be in the general election campaign. Although Clinton cast herself as by far the more experienced and qualified person to be commander in chief when campaigning against her rival for the Democratic nomination, Sen. Bernie Sanders, she often tried not to feed liberal suspicions that she is a hawk.

Clinton’s campaign celebrates each high-profile rejection of Trump by fellow Republicans and implicitly invites their support, but is leery of open courtship. Neither Mansoor nor several other Republicans opposing Trump said they have been contacted about supporting Clinton, although some plan to support her.

“I would support a random name in the phone book” over Trump, said Philip Zelikow, a University of Virginia history professor who was a State Department official in the George W. Bush administration.

National-security issues offer Clinton a way to play up her experience in contrast to Trump and appeal to people who probably would not vote for her otherwise, Clinton backers said. These include moderate Republicans and independents, but also suburban women turned off by Clinton but unwilling to support Trump, and some white men.

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Although Sanders is running close to Clinton in California, she is expected to clinch the nomination even if she loses the June 7 primary there. As in most of the country, the Democratic campaign in California has focused largely on domestic economic issues, but the state’s defense industry and military bases lend a backdrop for her speech.

The most recent Washington Post/ABC News poll in May showed Americans are largely split over whether Clinton or Trump would handle national-security issues better.

Slightly more said Clinton (47 percent) would do more to make the country safer and more secure than Trump (44 percent), but the difference is within the margin of error.

Among Democrats, 84 percent say Clinton would do more to make the country safer and a similar 83 percent of Republicans say the same for Trump. Independents shift toward Trump: 50 percent of them say that Trump would make the country safer and more secure; 39 percent say Clinton would.

Emily Guskin contributed to this report.