Speaker of the House Nancy Pelosi, D-Calif., arrives for a meeting of the House Democratic Caucus in the Capitol on Wednesday, December 4, 2019.

Smith would have been the highest-ranking Democrat in the House to publicly call for Pelosi to pass the articles along to the Republican-led Senate for a trial that could remove Trump from office.

In a tweet, Republican Sen. Ted Cruz chided Smith's about face: "Did somebody get sent to the Principal's office?"

Less than an hour before Pelosi was scheduled to hold a news conference, Smith tweeted: "I misspoke this morning, I do believe we should do everything we can to force the Senate to have a fair trial." If the speaker thinks withholding the articles will "help force a fair trial in the Senate," Smith added, "then I wholeheartedly support that decision."

Other Democrats have called on Pelosi to let the impeachment process to move to the Senate. She has not done so, trying to gain leverage in setting the rules for the trial.

Smith, a Washington state Democrat, had said on CNN that "I think it is time to send the impeachment to the Senate and let Mitch McConnell be responsible for the fairness of the trial."

House Armed Services Committee Chairman Rep. Adam Smith quickly reversed himself Thursday after saying Speaker Nancy Pelosi should send the articles of impeachment against President Donald Trump to the Senate.

But Smith wasn't the only Democrat to ease his pressure on Pelosi in the lead-up to her presser: Sens. Chris Coons, Dianne Feinstein, Richard Blumenthal and Joe Manchin all said Thursday that they think the decision is Pelosi's to make, NBC News reported.

The Democrat-led House impeached Trump in December on two articles — abuse of power and obstruction of Congress — related to his attempts to have Ukraine announce investigations into his political rival Joe Biden and his son Hunter.

After the articles passed the House, Pelosi said she was in no rush to send them to the Senate until McConnell, the Senate majority leader, shows he will carry out a fair process.

But more than three weeks later, her gamble to withhold the articles has yet to pay off in concessions from the Kentucky Republican. Earlier this week, he said he would move forward on Trump's impeachment trial without any Democratic support.

It's considered highly unlikely that two-thirds of the Senate will vote to convict and remove Trump from office. None of the 53 Republicans in the 100-member Senate have said they support convicting Trump.

Democrats have accused McConnell of planning to conduct a "sham" trial that will quickly exonerate Trump without a serious consideration of the information that continues to be unearthed since the president's impeachment in the House.

"I am concerned that Senator McConnell won't have a fair trial and I am with the Speaker that we should do everything we can to ensure he does," Smith said in another tweet Thursday. "Ultimately, I do want the articles sent to the Senate for the very simple reason that I want the impeachment process to go forward."

Still, some Senate Democrats, such as Dianne Feinstein of California, have already said Pelosi should pass the articles on to the Senate.

"The longer it goes on the less urgent it becomes," Feinstein said Wednesday. "So if it's serious and urgent, send them over. If it isn't, don't send it over."

Pelosi's office did not immediately respond to CNBC's request to comment on Smith's remarks. Pelosi was scheduled to hold a press conference Thursday at 10:45 a.m. ET. A spokesman for Smith did not immediately provide more comment.

The chambers have clashed over whether to guarantee testimony from witnesses.

Senate Minority Leader Chuck Schumer, D-N.Y., has called for four witnesses — including former national security advisor John Bolton and Trump's chief of staff Mick Mulvaney — to testify in the trial.

Bolton said Monday he would testify in Trump's impeachment trial if he received a subpoena. But McConnell has rejected Schumer's demand, saying instead that he wants the trial to closely resemble former President Bill Clinton's.

In that instance, senators debated whether to call witnesses after the trial started.

"We need to get folks to testify and we need more information ... but nonetheless, I'm ready," said Sen. Jon Tester, D- Mont., according to Politico.

"I don't know what leverage we have. It looks like the cake is already baked," Tester said.

Before his reversal on social media, Smith was asked Thursday morning if it was time for Pelosi to send the articles to the Senate.

"I think it is," he said. "I understand what the speaker was trying to do, basically trying to use the leverage of that to work with Democratic and Republican senators, to try to get a reasonable trial — a trial that would actually show evidence, bring out witnesses."

"But at the end of the day, just like we control it in the House, Mitch McConnell controls it in the Senate. I think it was perfectly advisable for the speaker to try to leverage that to get a better deal," he said.

Smith added: "At this point, it doesn't look like that's going to happen."