Republican presidential nominee Donald Trump is leading Democratic nominee Hillary Clinton 40 percent to 39 among likely Iowa voters included in a new Simpson College/RABA Research survey when the Libertarian and Green Party candidates are also options.

When matched up head to head in the poll taken Sept. 6 to Sept. 8, Trump leads with 43 percent over Clinton who sits within the three percent margin of error at 42 percent. Fifteen percent of those faced with a choice between Trump or Clinton chose “Not sure.”

Among those identifying as neither Republican nor Democrat and faced with the choice of Trump or Clinton, 40 percent chose Trump, 35 percent picked Clinton, and 24 percent were not sure. Evangelical Christians chose Trump over Clinton by an astounding nearly 30 percent margin, with 57 percent selecting Trump to just 28 percent for Clinton.

Just over half of respondents, 52 percent, expressed that they will be voting “In favor of my candidate.” Forty-one percent said his or her vote will be cast “Against another candidate.”

A majority, 57 percent, of those likely Iowa voters surveyed want to see third-party candidates included in presidential debates. Thirty-seven percent of those surveyed said they were not Republican or Democrat, but “something else,” while 32 percent were Republicans and 31 percent were Democrats.

Libertarian candidate Gary Johnson garnered 10 percent in the poll and Green Party candidate Jill Stein received three percent when matched up against Trump and Clinton. Eight percent of respondents chose “Not sure” when given a choice among the four.

The Real Clear Politics average of polls from Aug. 8 to Sept. 1 with the options Trump, Clinton, Johnson, or Stein shows Trump leading with 40 percent to Clinton’s 39 percent. This is consistent with the four-way matchup percentages Trump and Clinton received in this new Simpson/RABA poll.

Over half, 51 percent, of those surveyed held at least a college degree or higher and 60 percent identified as not evangelical Christian when asked whether they were or not. Over half of respondents were 30-64 years of age. Only 16 percent were 18 to 29 and the remaining 29 percent were 65 years of age or older. Fifty-three percent of respondents were male.

Eighty-eight percent of those who took the survey said they were “absolutely certain” they will vote, eight percent said “probably,” and four percent said “50-50.”

The survey was conducted using automated phone and internet surveys of 1,054 likely Iowa voters.

Follow Michelle Moons on Twitter @MichelleDiana