It seemed like an obvious political coup: scores of Republicans telling voters that a Hillary Clinton presidency would be safer than a Donald Trump one. Fifty of them signed an open letter warning that Trump “would put at risk our country’s national security.” Fifty more joined “Together for America,” a Clinton effort urging voters to “put country over party.” Then her team began exploring whether former secretaries of state, such as Henry Kissinger, might back her.

That, liberals warned, would be a step too far. And the prospect fed a perception that, with a contentious primary behind her, the Democratic nominee has returned to her old, hawkish ways and is again taking progressives for granted.

“What does Kissinger, a veritable war criminal, do to help win over these voters?” Alex Shephard asked in the New Republic.

Wrote Greg Grandin in the Nation: “Kissinger is a unique monster. He stands not as a bulwark against Donald Trump’s feared recklessness and immorality but as his progenitor.”

Wrote Isaac Chotiner in Slate: “Clinton must exercise the judgment that will be required of her in office. The prospect of Kissinger having influence in a Clinton White House is downright scary.”

The number of influential Republican officials saying that they can't vote for either Donald Trump or Hillary Clinton is growing as Sen. Susan Collins (R-Maine) pledges she won't vote for Trump. (Jenny Starrs/The Washington Post)

These progressives opposed the national security decisions Clinton made in the Senate and the State Department. They already felt that their questions about her were not answered in the primaries, which is why so many of them backed Sen. Bernie Sanders (Vt.). With his campaign in the rearview mirror, they see Clinton courting the sorts of Republicans they have worked against for decades — and it angers them.

[Clinton rolls out more GOP endorsements, announces new Republican outreach]

“It amuses me how Democrats who once found these people appalling are now cheering them as useful weapons in their glorious battle against Trump,” said Doug Henwood, a journalist who has written extensively about why progressives should not trust Clinton. “I’ve never been a big fan of Obama’s, but I’m already getting nostalgic for him. Hillary’s into all the stupid s--- he’s somewhat tried to avoid.”

As first lady, Clinton was viewed by Republicans as the most liberal member of a dangerously left-wing political family. But as a politician, she assembled a long list of differences with progressives, starting with her 2002 vote to allow U.S. military intervention in Iraq. Her narrow loss in the 2008 presidential primary was celebrated by Democratic doves, who believed they’d defeated a political establishment that too easily veered into war.

In 2015, when it appeared that Clinton would have a lazy stroll to the nomination, neoconservatives such as Robert Kagan suggested that she would be acceptable to Republicans and hawks. The rise of Sanders made the Clinton campaign skittish about such talk and unlikely to draw attention to praise from hawks.

At a February 2016 debate in Milwaukee, Sanders shamed Clinton for writing that Kissinger was a friend who “checked in with me regularly, sharing astute observations about foreign leaders and sending me written reports on his travels.”

“I happen to believe that Henry Kissinger was one of the most destructive secretaries of state in the modern history of this country,” Sanders said. “I am proud to say that Henry Kissinger is not my friend. I will not take advice from Henry Kissinger.”

Secret Service agents rushed to Democratic presidential nominee Hillary Clinton’s side at a rally in Des Moines, Aug. 10, stopping a protester who was running at the stage. Clinton didn’t stop speaking during the incident. (The Washington Post)

Although Trump’s romp through the Republican primaries rattled some supporters loose, the Clinton team hesitated to publicize the endorsements until Sanders’s campaign was over.

But on the final night of the Democratic National Convention, many Sanders delegates watched in agony as retired generals took the stage and endorsed Clinton. What Clinton’s campaign packaged as a popular front against a dangerous candidate, progressives saw as a needless swing to the right — one that empowered discredited foreign policy thinking.

“I don’t think Clinton will change her foreign policy because she’s receiving endorsements from Bush, Reagan and Nixon foreign policy officials,” said Glenn Greenwald, a Pulitzer Prize-winning co-founder of the Intercept. “I think she’s receiving those endorsements because they like what her foreign policy will be. That’s what worries me.”

Clinton’s post-convention polling surge may have reduced the leverage of progressive voters. According to a Washington Post-ABC News poll released Sunday, Clinton is winning 11 percent of Republican registered voters, similar to 13 percent for Barack Obama in August 2008. Just 14 percent of Democratic-leaning voters who wanted Sanders to win the primary do not support Clinton in a two-way race, compared with 30 percent of Clinton supporters who did not support Obama in August 2008.

Much of that, progressives say, is because of the real threat of a Trump presidency. Since conceding the primary, Sanders has relentlessly warned his supporters not to oppose Clinton’s campaign. (He declined through a spokesman to comment for this article.) Noam Chomsky, a left-wing intellectual who has criticized Kissinger’s foreign policy interventions for decades, said it still makes sense for voters to unite against Trump.

“I don’t like it, but it cannot give me pause, since the alternative is vastly worse, even on endorsements,” Chomsky said. “I don’t see the significance of neocons endorsing Clinton, probably not because they particularly like her but because they are concerned, sometimes terrified, about Trump.”

The conservatives abandoning the GOP over Trump have the same concerns.

“I’m speaking up so that when my grandchildren come home from college and ask me what I was doing in this election, I can tell them,” said Eliot Cohen, a co-founder of the Project for a New American Century who signed the “letter of 50” and planned to write in the name of Sen. Ben Sasse (R-Neb.) for president. “What I’d say to Sanders voters is: Can’t you accept the possibility that there are people you disagree with, but who are on your side this time?”

What neither the critics nor the endorsers can know is how the cross-party endorsements will affect a possible Clinton administration. After his 2008 victory, Obama’s decision to make Clinton secretary of state and appoint Clinton ally Rahm Emanuel as chief of staff angered many on the left. A “popular front” would allow some Republicans to share credit for a Clinton win. Unions that had once opposed Meg Whitman might see her discussed as a Cabinet member; anti-war activists who wore out shoes for Sanders might see neoconservatives considered for the State Department.

Cohen wasn’t thinking about that yet. “I wouldn’t be surprised if they give a few appointments here and there, but they’re going to pick mostly from their own bench,” he said. “As I said to a Democratic friend of mine in Aspen, you guys have got a lot of hungry mouths to feed.”