Nancy Pelosi, the US House speaker, might sympathize with the quote attributed to 19th century French politician Alexandre Auguste Ledru-Rollin: “There go the people. I must follow them, for I am their leader.”

For months following the release of the Mueller report, Pelosi had resisted pressure from within her caucus to begin impeachment proceedings against Donald Trump. But now, with more than two-thirds of Democrats in the House of Representatives favoring impeachment in the wake of Trump’s Ukrainian scandal, she has reversed herself and announced a formal impeachment inquiry.

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Pelosi had objected to impeachment mainly for political reasons. She observed that the Mueller report’s lengthy catalogue of Trump’s malefactions didn’t move public sentiment toward impeachment. She predicted that the Republican-dominated Senate would surely reject any articles of impeachment approved by the Democratic House, making the process appear to be merely partisan theater – a repeat of the 1998 impeachment proceedings mounted against President Clinton by House Republicans, which ultimately backfired and led to Democratic gains in that year’s elections. And she pointed out that impeachment proceedings might jeopardize the moderate Democrats who won Republican-held districts in 2018, allowing the party to regain control of the House.

But on 23 September, a group of seven first-term Democrats from those very battleground districts – all of whom had served in the military or defense and intelligence agencies, and six of whom had previously opposed impeachment – wrote an op-ed for the Washington Post declaring that “if these allegations are true, we believe these actions represent an impeachable offense”. Perhaps Pelosi, like those seven representatives, changed her mind because of the gravity of the new allegations, but their reversal certainly undercut her pragmatic political case against impeachment. And the Trump administration’s stonewalling of congressional investigations and refusal to abide by long-established norms of political conduct weakened Pelosi’s argument that methods short of impeachment could determine the truth of the allegations.

As a country, we now enter what the seven freshmen called “unchartered waters”, with an unprecedented political and constitutional crisis looming on the horizon. Impeachment proceedings could turn public opinion decisively against Trump, as with Richard Nixon and Watergate, perhaps leading to Trump’s premature departure from the White House. It’s also possible that impeachment could backfire on the Democrats, costing them control of the House and enabling Trump’s re-election in 2020.

If the latter scenario comes to pass, it may appear in hindsight that Pelosi’s caucus stampeded her into action against her better judgment. She had previously insisted that impeachment would require public support as well as backing from at least some Republicans. It’s too soon for polls to determine whether the latest revelations have shifted public opinion significantly, but so far only one Republican – Congressman Justin Amash of Michigan – has called for Trump’s impeachment, and he quit the party soon afterwards.

Chances are, however, that Pelosi is calculating that while impeachment will pour petrol on America’s already incendiary politics, Democrats stand to benefit from the conflagration. That’s largely because, unlike the Mueller report, the Ukrainian scandal involves what appear to be clear and comprehensible issues of corruption and threats to US national security and the electoral process. Trump already has admitted that he attempted to pressure a foreign government to dig up dirt to use against his political opponent, Joe Biden, and he may also have withheld military aid appropriated by Congress as further leverage. We don’t yet know exactly what is in the whistleblower’s complaint, and perhaps there will be no definitive evidence of an aid-for-dirt quid pro quo. But Trump’s actions will be difficult for Republicans running in 2020 to defend or distance themselves from.

The scandal likely will prove especially damaging in the suburban swing districts where Democrats won their House majority in 2018. Historically, the college-educated voters who predominate in such districts voted Republican because they considered it the party of stability and responsibility. There are many reasons why college-educated voters (particularly women) have been tilting against the Republicans in recent elections. But the Ukrainian scandal, however it plays out, will strengthen the sense among these voters that Trump considers himself to be above the law and constitutional restraints, and further erode the Republican party’s reputation as the law-and-order party. Undoubtedly this is why the statement issued by Pelosi and House majority leader Steny Hoyer emphasized that the decision to launch impeachment proceedings was “not a partisan matter, it’s about the integrity of our democracy, respect for the rule of law and defending our Constitution”.

Looking ahead to the 2020 election, the Ukrainian scandal is likely to put many Republicans running in those battleground districts in an untenable position. If they agree that the president’s actions merit condemnation, they will alienate the ferociously pro-Trump Republican base, but if they run as Trump defenders they will lose the moderate majorities in those districts.

The Democrats’ decision to begin impeachment proceedings has set the country on a course whose end is impossible to predict, but right now the political advantage seems to rest with Nancy Pelosi. In the wake of the Mueller report, she successfully prevented a drive toward impeachment that could have split the Democrats’ moderates and progressives. Although Ukraine-gate and the moderates’ resulting bolt toward impeachment may have forced her hand, she now presides over a unified caucus prepared to launch an investigation that may prove extremely dangerous for Trump and the Republicans.