He singled out her vote against the tax cut legislation.
Crowds gathered Wednesday morning in anticipation, with temperatures soaring to 89 degrees just before 4 p.m.
At a rally in Fargo to boost Republican candidate Kevin Cramer, the president said the incumbent Ms. Heitkamp betrayed her promise to be an "independent voice". But the president has seemed to have a warmer relationship with Heitkamp than other Democrats - he famously called her a "good woman" during his most recent North Dakota visit past year.
Trump planned to spend the night in the Milwaukee area.
Trump once again railed against the "elite" at a campaign rally in Fargo, North Dakota, echoing comments he made last week at a rally in Minnesota. Heidi Heitkamp in the midterms, calling her an incorrigible liberal who was out of step with the state's conservative values. He said he hopes Kennedy's replacement will serve for 40 or 45 years.
President Donald Trump's stepped-up campaign tour this week is taking him to North Dakota to help a Republican candidate who reluctantly entered a high-stakes Senate race and then questioned the support he has received from the White House.
Called to the microphone by the president, Cramer thanked Trump for rolling back federal regulations, cutting taxes and, "on behalf of the most vulnerable forgotten people, the unborn babies, thank you for standing for life". "All of that will affect producers in North Dakota". Republicans hold a 51-49 margin in the Senate.
Trump visits North Dakota as new economic data show the state falling behind its Midwestern neighbors amid concerns about an escalating trade war with China. "That's what you need and that is Kevin Cramer".
Trump tweeted before Wednesday's flight to Fargo that he was en route to "fully stand with and endorse" Cramer.
The protesters "really want anarchy", Trump said. "A vote for any Democrat is a vote for Schumer, Pelosi and Maxine - Maxine, she's a beauty". He highlighted a directive to create a sixth branch of the military, a Space Force.
Disparaging her from the stage - and no doubt pleasing Senate Republicans who have been urging him to get tougher with her - the president ticked off his grievances.
The president also dismissed the investigation into his campaign's ties with Russian Federation as a "hoax". But despite the festive mood, attendees spent most of their time inside being warned of outside threats, including "extremist Democrat politicians", the news media and the gang MS-13.
President Donald Trump says Supreme Court Justice Anthony Kennedy's retirement makes control of the Senate "one of the vital issues of our time".
Kennedy announced his retirement Wednesday, allowing Trump - who a year ago appointed Justice Neil Gorsuch - to fill a second seat on the Supreme Court.