"Over the past three weeks, I've travelled all over the country. But I'm here today in Michigan for a very important reason," Trump told the crowd in Port Huron on Sunday.
"I'm here to keep a jihadist Muslim Brotherhood member from becoming a United States senator."
The Republican president has been attacking El-Sayed's faith throughout the campaign and claiming his past statements and background show a sympathy for extremists.
El-Sayed, a former public health official, has repeatedly said he is loyal to the US and disavowed the Muslim Brotherhood, an Islamic movement with branches that the US government has designated as terrorist organisations.
The claims by Republicans have been denounced by Democrats as attacks on El-Sayed's faith and his Egyptian heritage. El-Sayed would be the first Muslim senator if elected.
Trump's return to Michigan reflects the importance of the race.
Republicans see an opening to flip a Senate seat as they try to hold onto the majority in the chamber.
El-Sayed faces Republican Mike Rogers, a former congressman backed by Trump, in the November 3 race to succeed retiring Democrat Gary Peters.
The race has taken on significance beyond Michigan, with Republicans casting El-Sayed as a symbol of a Democratic Party moving further to the left.
Progressives, meanwhile, see his candidacy as a test of whether their agenda can win in a battleground state ahead of the 2028 presidential election.
Trump on Sunday invoked the September 11, 2001, terrorist attacks, drawing a comparison between stopping El-Sayed from entering the Senate and the passengers who fought back against the hijackers aboard United Flight 93.
"Twenty-five years later, Michigan needs to stand up and fight, just like Todd did, to stop a jihadist from walking into the Capitol," Trump said, referring to Todd Beamer, a Michigan native who died aboard the flight.
Trump's visit to Port Huron follows former Democratic president Barack Obama's appearances on Saturday in Michigan and Ohio for his party's candidates.
It is Trump's latest stop on a packed campaign schedule to bolster Republicans on the mid-term ballot as the GOP is weighed down by voters' frustration with the economy and the war with Iran.
