The poll also found that Rep. Pete Hoekstra (R-Mich.) continues to have the inside path for the Republican U.S. Senate nomination — but that he will face a tough challenge against incumbent Sen. Debbie StabenowDebbie StabenowFight over water bill heats up in Senate Overnight Energy: Senate Dems set to fight water bill Senate Dems may block water bill over drought language MORE (D).

Hoekstra leads Detroit-area businessman Clark Durant 51-27 percent, a commanding lead.

"Hoekstra’s 24 point lead clearly shows Clark Durant has done too little, too late in this campaign," said Porn. "He should have been advertising and introducing himself to the public much earlier.”

But Stabenow holds a 49-35 percent lead in a hypothetical head-to-head with Hoekstra, a 3-point improvement from the June survey.

The presidential and Senate head-to-head polls carry a margin of error of 4 percent. The GOP Senate primary poll has a 3.5 percentage point margin of error.