MILWAUKEE (CBS 58) -- A Marquette Law School national survey released Thursday found former President Donald Trump remains the leading Republican candidate less than four weeks ahead of the first GOP presidential debate, which will be held in Milwaukee.

The poll found Trump ahead of Florida Gov. Ron DeSantis, 46% to 22%, among registered Republican voters and independents who said they lean Republican. Former Vice President Mike Pence had support from 7% of respondents, followed by former U.N. Ambassador Nikki Haley, who had 6%, and Sen. Tim Scott (R-SC), who 4% said they supported.

Other Republican candidates, including former New Jersey Gov. Chris Christie, business owner Vivek Ramaswamy and former Arkansas Gov. Asa Hutchinson all polled at 1%.

Trump's substantial lead over DeSantis is slightly greater than it was in Marquette's previous national poll, which came out in May. In that poll, Trump led DeSantis, 46% to 25%.

Democratic President Joe Biden saw his approval rate improve slightly, going from 39% in the May survey to 42% this month.

When asked to choose between only Trump and DeSantis, Republican voters were more evenly split. Trump led 54% to 46%, nearly identical to the May survey, when Trump led DeSantis 53% to 47%.

Trump's lead is a significant change from January's Marquette survey, when 64% said they favored DeSantis compared to 36% for Trump.

In a sign DeSantis' campaign is struggling less than a month from the August 23 debate at Fiserv Forum, CNN reported Wednesday DeSantis cut some of his staff. 

Trump has previously suggested he might skip the Milwaukee debate, which is being hosted by Fox News. His former chief of staff, Kenosha native Reince Priebus, told CBS 58 earlier this month he expected Trump to ultimately take the stage.

In hypothetical general election matchups, Trump and Biden were tied with each getting support from 50% of respondents. DeSantis led Biden 51% to 48%. 

However, those results were well within the margin of error, which was 4.2 percentage points among all voters. The margin was 5.8 percentage points when isolated to Republican voters.