WASHINGTON (NYTIMES) - After days of vote counting, former vice-president Joe Biden has amassed the 270 Electoral College votes he needed to secure the presidency.

All along, it was clear that Mr Biden's path to victory involved flipping a handful of states that then candidate Donald Trump won in 2016 while also retaining the states that Mrs Hillary Clinton, the Democratic nominee in 2016, won in that cycle.

Though there were many twists and turns, Mr Biden appears to have done exactly that.

Here is a quick look at the key states he won, and what we know about why he won them.

States Biden flipped

- Pennsylvania (20 Electoral College votes): Pennsylvania has been in the spotlight for months, and it was long considered a potential "tipping-point state" - the state where victory could determine the outcome of the entire presidential contest.

Elections officials had signalled all along that it would take time to count the votes there, and that a final tally would not be available on election night.

Mr Trump won the state by less than 1 percentage point in 2016, but polls had Mr Biden ahead in the run-up to Election Day.

And although it took days to learn who had won and the race was quite close, Pennsylvania indeed proved critical to the outcome.

As expected, Mr Trump jumped out to an early lead, thanks to ballots cast on Election Day, but Mr Biden clawed back slowly and eventually overtook Mr Trump as more and more absentee ballots were counted.

An initial look at county-level data suggests that Mr Biden ran ahead of Mrs Clinton's performance in 2016 in most parts of the state.

He showed particular strength in the suburbs around Philadelphia, an area that had been trending blue in 2016 and has only grown bluer since.

He flipped the state back into the Democratic column with 37,000 more votes than Mr Trump.

- Michigan (16 votes) and Wisconsin (10 votes): Four years ago, Mr Trump became the first Republican presidential candidate to win either Michigan or Wisconsin in decades when he defeated Mrs Clinton by roughly 33,000 votes in both states combined.

From the start of Mr Biden's presidential campaign all the way to Election Day, he and his team believed that rebuilding the Democratic "blue wall" in Michigan, Wisconsin and Pennsylvania would perhaps be the clearest and cleanest path to winning the White House.

Mr Biden also believed that the Democratic Party had to garner support from working-class and middle-class voters in northern industrial states - those from families like the one he grew up in.

That strategy paid off, as Mr Biden rode the support in Wisconsin's cities and suburbs to victory.

He was aided particularly by massive turnout in Dane County (home to Madison) and he ate into Mr Trump's margins in the Milwaukee suburbs.

In Michigan, a spike in turnout in Detroit and its wealthy suburbs, along with shrinking support for Mr Trump among middle-class voters, helped secure the state for Mr Biden.

About two-thirds of the predominantly white counties that backed Mr Trump in 2016 moved somewhat left four years later, and a greater share of voters in those counties backed Mr Biden than they supported Mrs Clinton in 2016.

Battleground states Biden kept

- Minnesota (10 Electoral College votes): Mr Trump came surprisingly close to a victory in Minnesota four years ago, and his campaign poured significant resources into the state this cycle in hopes of flipping it.

Nonetheless, Mr Biden maintained a comfortable if not insurmountable lead in Minnesota throughout much of the race, and polling there ended up being pretty spot on.

Mr Biden won comfortably, by about 7 percentage points. He ran up the margins in densely populated urban counties and made significant inroads in the suburban counties that Mr Trump won in 2016.

- Nevada (six votes): Mr Biden managed to hang onto Nevada, a state Clinton won in 2016, but it was a nail-biter that took days to decide.

Like Mrs Clinton, he did it by winning Clark and Washoe counties, home to Las Vegas and Reno, which account for over 85 per cent of the state's total votes.

His margin of victory in Clark County was down slightly from the margin in 2016, but it was up in Washoe.

- New Hampshire (14 votes): The Cook Political Report considered New Hampshire competitive with a Democratic lean, and it was tightly contested in 2016.

But Mr Biden won handily, by 7 percentage points, and the race was called relatively quickly.

Noteworthy states Biden lost

- Florida (29 Electoral College votes): Perhaps no state was more closely watched this cycle than Florida, and the results there almost immediately dashed Democrats' hopes of a blue landslide.

Mr Trump won the state by a significantly wider margin than he did in 2016, despite polling averages that showed him trailing before Election Day.

More than half of the counties in Florida swung further right than they did four years ago.

And although Mr Biden did make gains in some areas of the state, he vastly underperformed in Florida's most populous county, Miami-Dade, especially in precincts with a majority Hispanic population.

- Iowa (six votes), Ohio (18 votes) and Texas (38 votes): Optimistic Democrats hopeful for a blue wave saw Iowa, Ohio and Texas as potentially in play this cycle, but that proved to be wishful thinking.

Mr Trump thumped Mr Biden by significant margins in all three conservative-leaning states, winning them for a second time.

Of the three, Texas, where the President won by roughly 6 percentage points, ended up being the closest.

Even as some white voters in urban and suburban areas moved in large numbers toward Democrats, many Hispanic voters in the Rio Grande Valley shifted decisively towards Mr Trump.

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