Unlike the cases of Andrew Johnson and Bill Clinton -- the only two presidents who had been impeached until now -- and a third, Richard Nixon, who resigned before the House voted on impeachment, President Donald Trump's impeachment trial will focus on an abuse of power related to foreign policy.

Rarely do issues beyond our shores figure prominently in an election cycle, much less one where an impeachment trial is underway. Indeed, traditionally, domestic issues dominate presidential elections, with a tiny fraction of voters giving top priority to foreign policy concerns. But this year could be different. And here's why.

First, there's the intersection between foreign policy and domestic scandal

Not since Lyndon Johnson's Vietnam War debacle or Ronald Reagan's Iran Contra affair has a President been embroiled in matters of foreign policy that have created such controversy, dominated the political and media discourse and seemingly undermined his personal credibility and authority.

George W. Bush was heavily criticized for invading Iraq, yet he won reelection in 2004 without the tumult that now envelopes the Trump administration. Accusations of the Trump campaign's collusion with Russia (which the President vehemently denies) to manipulate the US political system and interfere in the 2016 elections have clung to this President like a barnacle to the side of a boat. (In his investigation of possible collusion, special counsel Robert Mueller found "insufficient evidence to charge a broader conspiracy.")

And the situation has been made worse by Trump's actions in creating a virtual zone of immunity from criticism for Vladimir Putin, reflected cogently by his veritable bowing before the Russian leader at their now famous press conference in Helsinki in 2018.

And now, President Trump is embroiled in yet another foreign policy scandal -- accused of abuse of power for using military assistance to Ukraine to leverage dirt on a political opponent (though the allegation is central to his impeachment trial, he denies that was his intent). The election is still 10 months away, and voters tend to focus on the here and now. But both Russia and Ukraine will now constitute key reference points in framing Trump's presidency. These issues are guaranteed to live on as Democrats make them key points of attack and Trump will have no alternative but to continue defending himself against them.

Second, there's Iran

The recent escalatory cycle with Iran, especially Trump's decision to kill Qasem Soleimani, head of the Islamic Revolutionary Guards Corps Quds Force, presents a potentially serious political problem for Trump. The President campaigned strongly on the issue of getting out of endless wars and not getting America into new ones. In places like Dubuque, Iowa that hadn't gone red in a presidential election since 1956 -- and in key battle ground states such as Michigan, Wisconsin and Pennsylvania -- Trump wooed a white working class constituency worried that Washington was focused too much on the rest of the world and not enough on America. He promised them "America first."

Now, all of a sudden the risk averse "let's get out of bad wars" President has deployed thousands of additional troops to the Middle East instead of withdrawing them.

Indeed, a recent Pew Research Center poll found that a majority of Americans believed Trump's recent actions on Iran had raised chances of a major conflict. More worrisome for Republicans, just one-third of Republican-leaning independents have a great deal of trust in what Trump says about Iran.

Still, for Iran to have an impact on the campaign, political experts say, the issue will have to remain in the national conversation and the Democrats would need to adopt a position fundamentally at odds with Trump's -- conditions the Iran escalation could easily meet. Indeed Democratic candidates have tended to bash Trump's Iran action.

Third, Rs and Ds really are divided on foreign policy

One of the reasons foreign policy has rarely played a role in affecting voter behavior is that the two parties have a hard time drawing major differences between their policies. Take for example one of the most contentious issues in US foreign policy: the Vietnam War. In the 1968 election, Vietnam did not have a decisive impact on voters because the public positions of Nixon and his running mate Hubert Humphrey on Vietnam were so similar: both called for grand reduction of US forces, both opposed unilateral withdrawal and both opposed escalation and invasion of northern Vietnam. Both also went to some lengths to keep their positions general and vague.

This type of agreement is all but extinct. There is very little consensus on anything in American politics these days, certainly not on foreign policy, on which Democrats have opposed Trump on everything from withdrawing from the Paris Climate Accord to immigration to withdrawal from the Iran nuclear deal to how he treated allies and cozied up to adversaries. Barring some foreign policy event that creates a unifying effect around the flag, foreign policy will remain a highly polarizing issue with voters taking their cues from their candidates. If anything, US-Iranian tensions seems to have demonstrated that the American public is reluctant to see the US in a major confrontation with Iran.

Whether unhappiness with Trump's approach to foreign policy and national security will create buyer's remorse for enough voters, especially independents, remains to be seen. But there's no doubt that the difference between Trump's view of the world and his Democratic challengers' views offer voters an unmistakable choice. And polls already demonstrate that a majority of Americans aren't where he is on many foreign policy issues.

The Unanticipated Crisis

In the 1950s, British Prime Minister Harold Macmillan was asked what might impact his tenure, he reportedly replied, "Events, Dear Boy, events."

Ten months is an eternity in foreign policy. Any number of potential crisis points might present themselves before election day -- Iran, North Korea, Russian influence in US politics or an unanticipated terror attack. None offer easy or simple solutions, and some carry real risks of missteps or miscalculation on the part of a mercurial President not known for carefully thinking through the costs and consequences of his actions. A foreign policy crisis between now and the elections would certainly push the issue of Trump's role -- and fitness -- as commander-in-chief to center stage, as it would highlight the experience (or perhaps lack thereof) of his Democratic opponent -- perhaps an advantage for the Democrats if the nominee turned out to be Joe Biden, who has long experience in foreign policy.

The bottom line is that just because foreign policy has rarely been a dominant influence in presidential campaigns and elections doesn't mean it won't play a greater role in this one. Trump's first term has shattered all kinds of conventional wisdom. Indeed, unlike in years past, this President's handling of foreign policy may well prove to be a controversial issue for voters, and one that's more relevant than ever in highlighting issues of presidential judgment and competency -- and America's role in the world.