Senate candidate Rep. Beto O'Rourke Beto O'RourkeJimmy Carter says his son smoked pot with Willie Nelson on White House roof O'Rourke endorses Kennedy for Senate: 'A champion for the values we're most proud of' 2020 Democrats do convention Zoom call MORE (D-Texas) on Sunday declined to respond to President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE's attacks on his character, saying he doesn't want to add to the "bitterness, name-calling [and] partisanship."

Trump in recent days has called O'Rourke a "flake" and a "total lightweight."

"I don’t know that it makes any sense to respond," O'Rourke told ABC News's Paula Faris in an interview that aired Sunday.

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"The bitterness, name-calling, partisanship … you can add more to it or you can stay focused on the future and why you did this in the first place," he said.

Rep. Beto O'Rourke says he won't respond to Trump's attacks: "The kind of bitterness and the name-calling and partisanship that has unfortunately defined so much of the national conversation, you can add more to it or you can stay focused on the future" https://t.co/OzZ6qEX9cI pic.twitter.com/vu4ZxiHQvA — This Week (@ThisWeekABC) October 21, 2018

Trump tussled with O'Rourke's Republican opponent, Sen. Ted Cruz Rafael (Ted) Edward CruzThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Cruz blocks amended resolution honoring Ginsburg over language about her dying wish Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Texas), throughout the 2016 presidential campaign, and even nicknamed him "Lyin' Ted."

Now, Trump has endorsed Cruz in the Texas Senate race, saying O'Rourke "is not in the same league."

"Ted is strong on Crime, Border & 2nd A, loves our Military, Vets, Low Taxes," Trump tweeted on Wednesday. "Beto is a Flake!"

Watched the debate last night & Beto O’Rourke, who wants higher taxes and far more regulations, is not in the same league with Ted Cruz & what the great people of Texas stand for & want. Ted is strong on Crime, Border & 2nd A, loves our Military, Vets, Low Taxes. Beto is a Flake! — Donald J. Trump (@realDonaldTrump) October 17, 2018

O'Rourke during a debate last week repeated Trump's nickname for Cruz, claiming the name "Lyin' Ted" has stuck because Cruz is dishonest.

The Texas representative during the ABC News interview said he now believes he should not have used that language.

Recent polls have shown Cruz leading O'Rourke by between 5 and 9 points, an exceptionally close race for deep-red Texas.

"We have one of the most simple, obvious strategies that I’ve seen employed," O'Rourke said on ABC. "We show up everywhere, all the time, for everyone."

O'Rourke also repeated that he does not ever intend to run for president.

"We've seen the consequence of a junior senator who leaves the state to pursue the presidency — leaves our priorities, our opportunities, our needs — behind," O'Rourke said, slamming Cruz for running in 2016.