A month after Nikki Haley dropped out of the Republican race, former President Trump is still dealing with a contingent of voters showing up to cast primary ballots for candidates who aren't him.

Why it matters: President Biden has more successfully unified his voters despite never facing a strong primary opponent and an organized protest vote over the war in Gaza.

• In 10 recent primary contests, more than one-quarter of GOP primary voters cast a ballot for a non-Trump candidate.
• "Joe Biden has a real golden opportunity to capture all those disaffected people who voted for Nikki Haley," said Arizona-based GOP strategist Barrett Marson.

Driving the news: In the key battleground state of Wisconsin on Tuesday, 20.8% of Republican primary voters cast a ballot for a candidate other than Trump.

• Haley, the former UN ambassador who suspended her campaign a month ago, drew more than 12%, or 76,000 votes, in Wisconsin, which Biden won by just over 20,000 votes against Trump in 2020.
• "Those are significant numbers," longtime Wisconsin Republican strategist Bill McCoshen told Axios.
• "Will those voters come home in November? I think it's possible they will, history suggests that most of them will, but I think it's also a signal to the Trump campaign that his pick for a VP could be very critical to bringing these voters back."
• Trump saw a larger share of protest votes in Wisconsin than Biden in the Democratic primary, where 8.3% of voters, or about 48,000, supported the "uninstructed" vote in protest of Biden's handling of the Israel-Hamas war.

The big picture: In another key presidential swing state, Arizona, Haley won about 18% of the vote in the GOP primary last month, despite having suspended her campaign about halfway through early voting.

• The "uncommitted" vote was not an option on the Democratic side in Arizona, but opponents of Biden's handling of the Israel-Hamas war encouraged voters to cast a ballot for Marianne Williamson as a protest.
• Williamson won 3.8% of the Democratic vote in Arizona.
• "That doesn't necessarily mean that 18% is filling in the bubble for Joe Biden in November, it's going to be up to Biden to message to those either moderate Republicans or right-leaning independents," Marson said.

In Michigan, another swing state, 13% of Democratic primary voters cast a ballot for the "uncommitted" movement last month.

• The Biden campaign said in a statement before the Tuesday primaries that "the president believes making your voice heard and participating in our democracy is fundamental to who we are as Americans."
• "He shares the goal for an end to the violence and a just, lasting peace in the Middle East. He's working tirelessly to that end."

What to watch: It's possible that some of the non-Trump votes in recent contests were from voters who cast a ballot while Haley was still in the race — and are planning to back Trump in November.

• But the sizable shares of protest votes could also be a sign of Trump's vulnerabilities ahead of an election that's likely to be decided by the margins in a few key swing states.
• "Donald Trump has made it clear he's not remotely interested in reaching out to independents, moderates or Nikki Haley supporters, who he's driven away with his extreme, unpopular agenda and his pledge to be a dictator on Day One," Biden campaign senior spokesperson Sarafina Chitika said in a statement.
• "President Biden, meanwhile, is meeting voters where they are and making it clear that all are welcome in his coalition to protect our freedoms and our democracy this November."

The Trump campaign did not respond to Axios' request for comment.

Go deeper: Biden's new ad targets Haley voters in swing states