For five weeks, the House of Representatives has heard public testimony about President Trump’s alleged efforts to pressure Ukraine into investigating former vice president and Democratic presidential candidate Joe Biden. This week, the House is expected to vote on two articles of impeachment based on that testimony.

Democrats hoped the hearings would increase public support for impeachment, but polling averages and daily tracking polls suggest public opinion is roughly the same as when House Intelligence Committee hearings began in November.

Several political commentators expressed concerns that the hearings’ revelations failed to sway public opinion. “What does it say about the threat to our democracy if you can’t win a debate based on facts?” MSNBC host Chuck Todd said.

But immobile public opinion is hardly surprising. Three factors, in particular, made shifting it extremely difficult.

1. Public opinion was already deeply polarized

It almost goes without saying: The public is profoundly divided over Trump’s presidency. Democrats and Republicans are more divided about Trump than past presidents, with an average 79-point difference in approval ratings between them. What’s more, that polarization is increasingly fueled by strongly held views on race, ethnicity, religion and gender.

So Americans’ views of the president are difficult to change. Trump’s approval rating in FiveThirtyEight’s polling average has held remarkably steady between 41 percent and 43 percent since February.

That polarization also makes it hard to change public opinion about the president’s conduct. Before the hearings began, most partisans had already made up their minds about whether Trump withheld military aid to Ukraine to extract political gain and whether such conduct is impeachable.

The left side of the figure above shows that, before the hearings, 90 percent of voters who supported Hillary Clinton in 2016 thought that Trump intentionally withheld aid to Ukraine to pressure its government to announce investigations into Biden and his son, Hunter, and that doing so was an impeachable offense. Trump voters, meanwhile, overwhelmingly thought that there was no “quid pro quo” and that even if there had been, it would not warrant impeachment.

The proceedings largely reinforced what most voters already believe. The right side of the figure shows that, after the hearings ended, Trump and Clinton voters were just as divided as before.

2. Congressional Republicans disputed the evidence and testimony

The hearings reinforced partisan polarization in large part because congressional Democrats and Republicans presented the American people with two “irreconcilable realities.”

Democrats and their witnesses suggested that the president abused his power by withholding congressionally approved aid to Ukraine to pressure a vulnerable ally into publicly announcing investigations of Trump’s political rival. Congressional Republicans on the House Intelligence Committee, however, were united in arguing that the hearings offered no evidence of presidential misconduct.

A long line of social science research shows that when political elites are this sharply divided, the public follows their lead. Partisan messaging is so powerful that Americans tend to adopt their party’s standpoint even when that position runs counter to science and objective facts.

As long as congressional Republicans remain unified in their defense of Trump’s actions, few rank-and-file partisans will change their opinions.

3. The most easily persuaded pay the least attention

Although partisanship and partisan messaging locked most Americans into their views before the hearings, a sizable minority were undecided on impeachment.

Public opinion is often difficult to change precisely because the most easily persuaded people pay the least attention to the news. This was certainly true for the impeachment hearings.

In two separate polls — one conducted after the first day of the impeachment hearings and the second fielded after the Intelligence Committee hearings ended — YouGov/HuffPost asked its respondents how closely they were following the impeachment proceedings.

The graph above shows that respondents who were undecided on impeachment in those two polls (14 percent and 13 percent, respectively) were much less likely to have heard about the impeachment hearings. In fact, a majority of these impeachment undecideds said, in both surveys, that they had heard nothing at all about hearings.

Occasionally, a news story is so prominent that even Americans who don’t pay much attention to politics are exposed to it. This happened in 1973-1974, when upward of 80 percent of Americans tuned in to at least some of the Watergate hearings on prime time. But because audiences now have scores of choices about what to watch, listen to or read, only a small and politically dedicated fraction of Americans watched the hearings.

And because the hearings didn’t reach the most easily persuaded Americans, their partisan interpreters reinforced preexisting and polarized views.

The result of those three factors? Stalled public opinion on impeachment.

Read more TMC analysis of the impeachment inquiry: