The president also riffed on (unfounded) allegations of voter fraud in California, the development of "clean, handsome coal" in West Virginia, and what he called the need for Republican wins in the November congressional elections.
"We have very weak laws because of the Democrats", he said.
This is President Trump's third visit to Southern West Virginia. Vice President Mike Pence - in words uncharacteristically harsh for the even-tempered vice president - railed against Manchin when he was in the state for a GOP conference earlier this year. Among them: "What will the 200,000 West Virginians do when they lose health care coverage".
Manchin, who is considered vulnerable to a Republican challenge in the deep red state, is running for re-election in November.
Trump was in White Sulphur Springs, West Virginia, on Thursday for what was promoted as a tax cuts roundtable, however, he literally threw out the script and opted for a more classic Trump off-the-cuff opening statement, which included calling out the state's Democratic senator.
But instead of spending much time on taxes, Trump belabored the many grievances against immigrants and USA immigration law that he voiced during his campaign and into his presidency.
"You've made a huge difference", said Terry Dotson, a president, chairman and CEO of Worldwide Equipment who took part in Thursday's roundtable. They always like to say, 'Oh that's a conspiracy theory.' Not a conspiracy theory, folks.
He repeated his unfounded and unsubstantiated claim that "millions and millions of people" voted illegally in 2016.. "And yesterday it came out where this journey coming up, women are raped at levels that nobody has ever seen before". He told the audience that they would get "a chance to get a senator that's going to vote our program".
That's according to Homeland Security Secretary Kirstjen Nielsen.
President Donald Trump has signed a memo clearing the way for the deployment of National Guard troops to the border. Both praised the president and his agenda at the event.
In a Thursday morning tweet, the president credits "the strong immigration laws of Mexico and their willingness to use them".
As he has done before, Trump conjured images of violence and suffering when he described the perils of illegal immigration, though statistics show that immigrants commit crimes at a lower rate than citizens.
Homeland Security Secretary Kirstjen Nielsen says she's been working with governors of the southwest border states to develop agreements on where and how many Guardsmen will be deployed.