Big-name Republican endorsers of Vice President Kamala Harris are testing just how many disgruntled GOP voters are up for grabs in her race against a polarizing former President Donald Trump.

Former Wyoming Rep. Liz Cheney, a member of pre-Trump GOP royalty, became the latest and most prominent Republican to back Harris Wednesday. Harris also has endorsements from former Rep. Adam Kinzinger, R-Ill., and hundreds of local Republican officials to try to puncture what her campaign views as Trump's soft underbelly with Republican voters who are uncomfortable with the former president's brash and unorthodox brand of politics.

The campaign's consistent outreach is just one part of Harris' overall path to Election Day, but now, with no bigger names left on the table for support, the vice president will likely find out if there's more support to be had from dissatisfied Republicans -- or if she's already maxed out.

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"The Kinzinger/Cheney endorsements are designed by Democrats to make Republican voters who are alienated by Trump's dishonesty, bad character, felony conviction, public policy ignorance, etc., feel better about not just abstaining but actively showing their displeasure by voting for Harris," said former Rep. Mo Brooks, R-Ala., a conservative six-term lawmaker who clashed with Trump during his 2022 Senate race.

"The number of Republican voters swayed by Republicans who endorse Harris is relatively small," he added. "But in an otherwise close race, it could be the difference between winning and losing."

Cheney, a rising star in yesteryear's GOP who became both an outcast and the face of the anti-Trump Republican flank after the Jan. 6 insurrection, said Wednesday in red-leaning North Carolina that she would back Harris out of fear for American democracy. On Friday, she said her father, former Vice President Dick Cheney, would do the same.

"I felt that it was a particularly important discussion to have for the first time in North Carolina, where, you know, one of the questions that that I hear from Republicans who know that they would not support Donald Trump again, but, you know, have sort of said, well, maybe I'll just write someone in, " she said on Friday. "And I think that, particularly when we're talking about states where we know it's going to be close, where the election will be decided, we don't have that luxury and I think it's really important to recognize the nature of the choice that we have," she said.

The endorsement is just one part of Harris' strategy to win over fence-sitting Republican voters.

The party sees an opening for expansion with wayward Republicans, particularly after hundreds of thousands of voters bucked Trump in the GOP primary to vote for former South Carolina Gov. Nikki Haley, including after she ended her campaign. And Trump has demonstrated little appetite for those who opposed him, saying at a town hall Thursday, "I don't want that person" when asked about people who didn't cast a primary ballot for him.

Democrats had a parade of Republican speakers, including Kinzinger, speak at their convention last month. Harris has said she would appoint a Republican to her Cabinet. A network of local officials and former Trump administration officials have hit the trail to stump for Harris. And the campaign has dumped millions of dollars in advertising to suggest to Republicans that they have a place in the modern Democratic Party.

To be certain, Harris is not putting Republican converts at the heart of her strategy, recognizing the limited opportunity challenging a GOP largely in Trump's thrall. But Democrats insist that winning over a sliver of protest voters can pay dividends -- and that big names like Cheney and Kinzinger could provide a permission structure for some Republicans to punch their ticket for a Democrat this November.

"We know that there are a number of Republicans who are in a similar situation. They're not never Trump, necessarily. Some may be, but they may not know how they feel yet about Vice President Harris," said Democratic strategist Karen Finney. "This kind of endorsement sends a signal to those voters that there is someone else who is high profile, but who thinks like they do about Trump and has similar concerns, and who they respect."

It's unclear precisely how much Cheney will be out on the trail converting Republicans, but there is already a playbook for how she can throw her weight around.

In 2022, she put $500,000 into an Arizona ad hammering Kari Lake and Mark Finchem, the Republican candidates for governor and secretary of state and vocal election deniers, saying that "I don't know that I have ever voted for a Democrat, but if I lived in Arizona, I absolutely would."

Both Lake and Finchem went on to lose their races.

But statewide races in a midterm year are not the same as a historic presidential election with a Republican candidate who sucks up an unparalleled amount of political oxygen.

Polling has shown Harris stuck in the single digits with support among Republicans, and Trump has been a public figure for years, including nearly 10 since he launched his first campaign, leaving perceptions about him cemented enough to withstand the new endorsements.

"My instinct is that those [Republicans] and [independents] aren't moved by that -- they are already on board with Harris," said Chuck Coughlin, a strategist in Arizona who left the GOP in the Trump era.

"Anyone who is already voting against Trump because they hate Trump is already there, GOP pollster Robert Blizzard agreed. "They aren't moving against him because a random former congressman says they are too."

Other Republicans said voters might not be persuaded by Cheney and Kinzinger's dire warnings about the threat Trump poses to democracy given that, despite the three-year-old Jan. 6 attack on the U.S. Capitol by a pro-Trump mob, guardrails held.

"I think Kinzinger and Cheney aren't that persuasive to fence sitting or 'Haley' voters. They typically do not accept the apocalyptic framing they use that Democracy ends with Trump. I think the most persuasive are those that worked with him like [Jon] Kelly, [Mark] Esper, [Henry] McMaster, [John] Bolton etc. who can say they worked with him and he's unfit," said anti-Trump GOP strategist Rob Stutzman, referencing former Trump administration officials with national security backgrounds.

And with all the work and money Harris has already put into appealing to Republicans, some operatives speculated that the vice president's ceiling on GOP support may have already been met.

"It certainly helps. But I'm not sure their endorsements are leading indicators of the Never Trump movement, instead they seem more to be lagging indicators of the movement," said former Florida Rep. David Jolly, who left the GOP over disagreements with Trump. "The real catalyst to the strengthening coalition has been Harris herself."

Trump's campaign, for its part, appeared unconcerned about the recent Cheney endorsements.

When asked what the campaign thought of Cheney announcing she and her father would vote for Harris, campaign spokesperson Steven Cheung replied, "who the f--- is Liz Cheney?"

ABC News' Isabella Murray contributed to this report.