A Washington-based conservative group wants Utah constituents of Sen. Mitt Romney to know it thinks he should oppose voting for witnesses in President Trump's Senate impeachment trial.

FreedomWorks took out a full-page ad in the Salt Lake Tribune on Thursday urging Romney, 72, to vote "no" on witnesses, which would likely include former Trump administration national security adviser John Bolton, discussing the president's interactions with Ukrainian President Volodymyr Zelensky. Senate Majority Leader Mitch McConnell, a Kentucky Republican, has been pushing to avoid witnesses and end the impeachment trial as soon as Friday.

"Senator Romney, don't destroy the integrity of our institutions. The House had its chance to make the case. Vote for no new witnesses! It's time for the Senate to end the impeachment sham and get back to work," the ad says.

In a statement, FreedomWorks President Adam Brandon said, “Democrats had their chance to call and question witnesses during their impeachment hearings but instead chose to pass their sham articles of impeachment at breakneck speed in the House."

[Read more: 'Wants to appease the Left': GOP senator blasts Mitt Romney for being open to calling impeachment trial witnesses]

Romney, a former Massachusetts governor and the 2012 Republican presidential nominee, has consistently supported calling Bolton to testify. Most of his fellow Republican senators are against it, saying it will trigger a slew of other witness demands from both the president’s legal team and the House impeachment managers, leading to a longer, drawn-out Senate impeachment trial.

Trump is charged with two articles of impeachment, obstruction of Congress and abuse of power, over a phone conversation he had with Zelensky related to military assistance to the country, and whether Trump conditioned the aid on Zelensky launching an investigation into former Vice President Joe Biden and his son Hunter Biden.

Members of the Senate are expected to continue submitting questions to Chief Justice John Roberts for eight hours Thursday. Senate Republicans hope to wrap up the trial by the end of the week.