A few decades ago, Republicans enjoyed a so-called lock on the electoral college. Later it was Democrats and a substantial blue wall of states that seemed to give them the edge in presidential races. Tuesday's midterm results underscored that, for now, those days are gone. Neither party can claim a clear advantage in the arithmetic that will decide who will win the White House in 2020. Voters delivered divided government to Washington, ousting the Republican majority in the House while reinforcing the GOP majority in the Senate. Those results, state by state and district by district, framed the geographic and demographic challenges for both President Donald Trump and whoever becomes the Democratic presidential nominee two years from now.

Tuesday's results highlighted the fact that the focal point of the struggle for electoral superiority over the next two years and probably beyond will be in the suburbs. Democrats dominate the big urban centers and Trump, heading a reconstituted Republican Party, has tightened the GOP's grip on rural America.

That leaves the one place of true competition the suburban voters, many of whom have long favored Republicans but who staged a revolt against the president Tuesday by voting for Democratic candidates.

The road to the White House ultimately depends on a handful of states.

Two years ago, Trump secured his victory by winning two big prizes, Ohio easily and Florida narrowly, and then carrying Wisconsin, Michigan and Pennsylvania by the thinnest of margins — less that one percentage point in each state.

On Tuesday, voters in Wisconsin, Michigan and Pennsylvania backed Democratic candidates for both governor and the Senate; in Wisconsin and Michigan, they reversed eight years of GOP rule in the governor's mansion.

What that means for 2020 is not entirely clear, given that two of the senators and one of the governors elected were incumbents.

At the least, the Democratic victories provided a morale boost, and in demonstrating the coalition needed to win, may represent at least a symbolic roadblock to the president as he maps his 2020 strategy. But party strategists acknowledged Wednesday that much work remains to be done in those Midwest battlegrounds.

Tuesday's results in Ohio and Florida serve as a reminder to Democrats of challenges that the party's nominee could face in two states that have provided some of the most hard-fought presidential contests of the past two decades.

In Ohio, Democratic Sen. Sherrod Brown won his re-election bid, offering Democrats one model for winning a competitive state with a progressive record and message. But in the governor's race, Republican attorney general Mike DeWine defeated Democrat Richard Cordray.

Trump won Ohio by nine points two years ago, and DeWine's victory continued the GOP's general dominance in statewide races.

Ohio has long been a fiercely contested state, though one with a slight Republican edge. That may have shifted recently to give Republicans a greater advantage than in the past.

Florida did what it always does, delivering races as close as any in the country.

Republican Rep. Ron DeSantis claimed the governorship over Democrat Andrew Gillum, the mayor of Tallahassee, by less than a point, while the Senate contest between Democratic Sen. Bill Nelson and Republican Gov. Rick Scott is heading for a recount.

As one Democrat put it on Wednesday, "We have not figured out Florida, which is a problem." By that the strategist, who spoke on the condition of anonymity in order to speak about his party's problems, meant that Democrats continue to underperform among white voters, especially those without college degrees, and cannot count on being able to motivate minority voters in numbers needed to turn narrow losses into narrow victories.

Results elsewhere speak to changes underway in other states that traditionally figure into the competition in presidential races.

Democrats again scored well in Virginia, which has moved from a solid red state to a purple state to one with an increasingly blue tint. Democrats picked up three House seats, benefiting from strong support from suburban voters.

In Colorado, another purple state, Democrats maintained their hold on the governor's mansion and picked up a suburban House seat.

In Nevada, another purple state, Democrats defeated a Republican senator (Dean Heller) and captured the governorship.

Democrats have had their eye on other Sun Belt states in recent presidential elections, namely Georgia, Arizona and Texas. After Tuesday's results, which saw Democrats narrowly lose a Senate race in Texas and fall behind in the Georgia governor's contest and the Arizona Senate race, party strategists said they worry that 2020 might be too soon to put them into the Democratic column.

Yet House results show the changing patterns in suburban America, even in some places where the statewide contests went against them.

Democrats built their new House majority with victories in suburban territory where Republicans have enjoyed longstanding support.

The shifts from Republican to Democrat were substantial in many of those districts. But also striking was how many of the House races were decided by margins of five points or less.

A rough analysis of the races showed 18 Democrats who either were victorious or were leading on Wednesday won their races by five percentage points or fewer.

Republicans will be aiming to take those back in two years. But a measure of how much bigger the overall Democratic House majority could have been is that about two dozen Republicans won or were leading led by similarly narrow margins of five points or fewer.

Trump's rhetoric, his style and his divisiveness clearly cost the Republican Party on Tuesday in these suburban House districts and will complicate his prospects for re-election in two years.

But Chicago Mayor Rahm Emanuel argued Wednesday that Democrats must build an urban-suburban coalition through issues like health care and education and thereby prevent the president from driving a wedge between the two constituencies on other issues.

Emanuel, who as a member of Congress oversaw his party's takeover of the House in 2006, said he has no doubt that what took place in House races was a rejection of the president, especially given the strength of the economy and the degree to which congressional district lines favor the Republicans.

"But it's not a realignment," he added, "unless we do the hard work, both in 2020 and beyond."