Sen. Doug Jones (D-Ala.) is questioning in a new op-ed whether a majority of senators will "pursue the truth" in upcoming impeachment trial of President TrumpDonald John TrumpUS launches airstrikes targeting Iran-backed militia in Iraq, Syria Trade, interest rates top finance fights for 2020 Five health care fights to watch in 2020 MORE.

Jones, considered the most vulnerable Senate Democrat up for reelection next year, called for a “full, fair and complete trial” with all necessary witnesses and documents to fulfill the Senate’s “solemn constitutional duty.”

“I fear, however, that we are headed toward a trial that is not intended to find the whole truth,” he wrote Monday in The Washington Post. “For the sake of the country, this must change.”

Jones said the current amount of evidence may be enough to “make a judgment” but called it “clearly incomplete.” 

The senator requested at least four witnesses with “direct knowledge” of the White House's withholding of military aid to Ukraine to testify: former national security adviser John BoltonJohn BoltonDemocrats worry impeachment acquittal will embolden Trump Putin's next aggression Senate GOP wants speedy Trump acquittal MORE, acting chief of staff Mick MulvaneyJohn (Mick) Michael MulvaneyDemocrats worry impeachment acquittal will embolden Trump Warren: 'If there's a lawful order on a subpoena, then I assume' Biden would comply Pelosi gets under Trump's skin on impeachment MORE, senior adviser to Mulvaney Robert Blair and associate director for national security at the Office of Management and Budget Michael Duffey.

“Let me be clear: I do not know what their answers would be, but I want to hear from them, and so should every senator and every American,” Jones wrote. “We cannot allow the full truth to evade this trial only to be revealed in some future memoir or committee hearing.”

Trump is accused of pressuring Kyiv to announce investigations that would have benefited him personally ahead of the 2020 election while also withholding hundreds of millions of dollars in congressionally approved assistance to Ukraine. Earlier this month, the House approved two articles of impeachment against him: abuse of his office and obstruction of Congress.

Jones called for Bolton to answer under oath questions about his role in withholding the aid, his leaving of his position earlier this year and his response to testimony from Fiona Hill, the former National Security Council senior director for Europe and Russia. 

Republicans see Jones and Sen. Joe ManchinJoseph (Joe) ManchinGOP predicts bipartisan acquittal at Trump impeachment trial Susan Collins set to play pivotal role in impeachment drama Lawmakers pile on the spending in .4 trillion deal MORE (W.Va.) as the most likely Democrats to break party lines and vote to acquit the president.