Political scientist Michael Cornfield said on Wednesday that the Democratic nominee in 2020 will ultimately need to focus on an issue other than President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE on the campaign trail.

"The presidential campaign trail is littered with the carcasses of people who tried to defeat Donald Trump by running against him, instead of for something, on their own issue," Cornfield, the director of the GW Politics Poll, told Hill.TV's Joe Concha on "What America's Thinking."

"Whoever wins the Democratic nomination will be the one who articulates a vision, and has an appealing personality, but above all, has warned that American people that, 'hey, we're ignoring this. We need to take care of this,'" he continued.

Cornfield's comments come hours after Republicans maintained control of the Senate, but lost their majority in the House in the midterm elections.

The elections were widely interpreted as a referendum on President Trump's first two years in office.

Trump on Wednesday appeared to be happy that Republican House incumbents who distanced themselves from him, such as Reps. Carlos Curbelo (Fla.), Barbara Comstock (Va.) and Mia Love (Utah), were all defeated.

The conclusion of the midterms has led to further speculation on who will challenge Trump in 2020.

Potential challengers include former Democratic Vice President Biden, as well as Sens. Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (D-Mass.) and Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.).

— Julia Manchester