CNBC's "Millionaire Survey," which in March surveyed 750 people with $1 million or more in investable assets, found that in a head-to-head contest between Clinton and Trump, 44 percent of millionaires would vote for Clinton in November, compared with 31 percent for Trump.
Clinton also has a more committed voter base among millionaire voters. When asked to rate their support level for candidates — with zero being "no support at all" and 100 being "strongly support" — Clinton received a total rating of 38, while Trump earned a 28. Among Democrats, Clinton earned a support rating of 78, while Trump rated only 50 among Republicans.
Yet 15 percent of millionaires remain undecided on their vote in November. And millionaire support for Trump — typically viewed as the favorite of blue-collar workers — is growing, with his share of millionaire voters more than tripling over the past six months. Last fall, CNBC's Millionaire Survey showed that only 9 percent supported Trump in a crowded field of Republican candidates.
When asked about the source of Trump's popularity, the largest number of millionaires chose "he is an alternative to traditional politics" as the chief reason.
"Trump may be able to pull in more affluent voters as we get closer to November," said Catherine McBreen, managing director of Spectrem Group, which conducts the survey with CNBC. "Trump may have started talking about the working class, but he's smart enough that he knows he's going to have to pull in others."
The CNBC Millionaire Survey skews slightly more Republican than national polls, making the level of support for Clinton even more surprising. The survey polled 264 Republicans, 217 Democrats and 254 Independents.
Clinton won the support of 90 percent of Democratic millionaires, 42 percent of Independents and 8 percent of Republican millionaires. Trump won the support of 61 percent of Republican millionaires, 22 percent of Independents and 4 percent of Democratic millionaires.
Fully 8 percent of all millionaires said they "will not vote" if Clinton and Trump are the two choices in November.
When asked about the most important issues in the presidential election, millionaires ranked "foreign policy/world leadership/combating terrorism" as the top issue. They ranked "political gridlock" second, followed by "taxes/government spending."
When asked about the most important criteria for a presidential candidate, the largest amount — 47 percent — said "someone who is trustworthy and honest," followed by "someone who is a unifier (15 percent). Only 11 percent chose "someone who has the same values I do."
Of course, in an era of populist politics, neither Clinton nor Trump want to be seen as the millionaire favorite. Yet millionaires vote at a high rate and are an important source of cash for candidates.
More than one in five millionaires has given money to a candidate in the past year. Additionally, 11 percent have donated to a political action committee, with another 10 percent planning to do so in the future.
When it comes to giving to individual candidates, 46 percent gave to Democrats and 38 percent percent to Republicans. As for giving to PACs, 38 percent gave to Republican PACs, while 32 percent gave to Democrats.