Washington (CNN) Republican Sen. Mitt Romney of Utah said on Wednesday that he voted for a Republican in his state's presidential primary election but wouldn't say if the candidate he voted for was President Donald Trump.

Asked by CNN if he did not vote for Trump, Romney responded, "Not saying," after he said that he had voted for "a Republican" in the Utah primary.

Trump is expected to easily secure the Republican Party nomination in his 2020 reelection bid, but there are some long-shot GOP challengers have still tried taking him on.

Former Massachusetts Gov. Bill Weld and former US Rep. Joe Walsh are both listed as Republican Party candidates in an online state directory of presidential candidates who have officially declared their candidacy in Utah. Trump is on the list, as are several other lesser-known candidates.

Walsh announced that he was ending his longshot challenge against Trump earlier this year after suffering a crushing loss in the Iowa GOP caucuses in which he received only 1% of the vote.

Romney and Trump have had a contentious relationship that grew increasingly tense during the Senate's impeachment trial of the President after the Utah Republican broke ranks with the party to vote to convict the President on the charge of abuse of power.

The President responded by attacking Romney as a "failed presidential candidate."

Romney isn't the only Republican senator keeping quiet about whether they voted for Trump in the 2020 primary election.

GOP Sen. Susan Collins, who is up for reelection in Maine in 2020, would not say on Tuesday if she voted for President Donald Trump in her state's primary. She twice declined to answer the question.

"I already answered that question," Collins told CNN when asked if she supported Trump's reelection bid.

Collins' spokesperson referred CNN to earlier comments the senator made when she was asked if she is supporting Trump. But in that interview, she didn't directly answer when asked if she backed the President's 2020 bid.

"I have voted by absentee ballot, just to make sure that I voted," Collins told WCSH in Portland, Maine

"And I would note that it's on the Democratic side that there are eight candidates, and my likely opponent as well as the governor and many other Democratic officials have not said who they are going to choose in what is a contested Democratic ballot. I'm focused on my job and also on my own campaign, and I'm just not going to get involved in presidential politics."