More than half of Americans think President Donald Trump's behavior warrants his removal from office and 57% think he's committed an impeachable offense, according to a poll released Friday morning – unwelcome news for a president facing a Senate trial that could begin this month.

Further, the survey found that 57% of Americans want the Senate to hear from new witnesses to shed light on Trump's conduct, a demand Democrats have made – so far without success – of the GOP Senate majority.

The FiveThirtyEight/Ipsos poll reflects a partisan divide over Trump that has been consistent in surveys by a wide variety of pollsters throughout the investigation and December impeachment of the president, with the vast majority of Democrats wanting Trump removed and the overwhelming number of Republicans opposing the action.

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But the trend is not a positive one for the embattled president, who is now grappling with a foreign policy crisis as well after he ordered the killing of an Iranian general Thursday, provoking vows of retaliation from the Iranian government. A poll taken by Reuters/Ipsos in the hours after Trump's impeachment found just 42% in support of removing Trump from office. And aggregate polling tracked by FiveThirtyEight since October shows support for removing the president hovering at 47%.

The shift in the poll numbers isn't dramatic, FiveThirtyEight's analysts wrote in a summation of the survey, but "because it's proven exceptionally difficult so far to change Americans' minds about Trump's impeachment, any shifts – even if they're relatively minor – are worth noting. This doesn't mean that the expected outcome of the Senate trial will change, but there is evidence that some of those potentially persuadable respondents have been drawn more firmly into the pro-impeachment camp over the course of the process."

Trump is a uniquely polarizing figure in American politics, with polling on his job performance, overall approval rating and impeachment falling largely along party lines. But the latest poll is troubling for the president as he seeks to expand his fiercely loyal base enough to win reelection in November.

The two presidents facing impeachment in the modern era were faring better in public opinion polling at this stage in their struggles to stay in office. After the explosive Watergate hearings, for example, only 19% of Americans favored removing Richard Nixon from office. Even in July of 1974, when the Supreme Court ruled Nixon had to hand over damaging tapes, support for ejecting Nixon from office was below 50%, according to Gallup polling compiled by the Pew Research Center . The number hit 57% as Nixon, facing certain impeachment, resigned the presidency.

President Bill Clinton, impeached for lying to a grand jury about his relationship with a White House intern, never suffered such a public opinion backlash. In the week of Clinton's Senate trial, during which he was acquitted, a CNN/USA Today/Gallup poll found that nearly two-thirds of Americans did not want Clinton removed from office.

The Senate trial – still unscheduled, as Democratic House Speaker Nancy Pelosi of California has refused to send the articles of impeachment to the Senate – has already been marred by charges of bias. The Senate majority leader, Kentucky Republican Mitch McConnell, has said outright that he is working with the White House defense team, enraging Democrats who say it's akin to a criminal court juror colluding with the defense attorney.

Republicans argue that Democrats have also already made up their minds, making any trial a sham.

Pelosi is withholding the articles of impeachment until she is certain "what sort of trial the Senate will conduct" – meaning, whether the Senate will hear from witnesses. New details have emerged since the House impeachment about the president's involvement in withholding critically needed, congressionally approved aid to Ukraine. McConnell has countered that it is up to the Senate to make those decisions.

The Ipsos poll's findings that a strong majority, 57%, believes new witnesses should be called to testify appears to buttress Democratic demands. But the picture is more nuanced, since some people would like former Vice President Joe Biden and his son Hunter to testify – something Democrats say has nothing to do with the charges against Trump.