Democrats regained control of both chambers of the United States Congress in this year’s midterm elections, with a wave of victories that party leader Hakeem Jeffries likened to an “American tsunami”.

But as of Wednesday night, two US Senate races remain undecided.

A cliffhanger in the southeastern state of Mississippi and a scandal-plagued Alaska race are the final Senate contests to be tallied.

As it stands, the Democrats will usher in the 120th Congress on January 3 with a majority of at least 54 seats in the 100-member Senate.

If they win both Mississippi and Alaska, they will need fewer Republican breakaways to avoid the filibuster, a means by which the minority party can indefinitely stall legislation in the chamber. Filibusters can be avoided with a 60-member vote.

So, why are Alaska’s and Mississippi’s Senate races still up in the air? Al Jazeera breaks it down with this brief explainer.

What’s the hold-up in Mississippi?

The Republican stronghold of Mississippi was an unlikely Senate battleground this year.

The last time the state voted for a Democratic president was in 1976. The last time it elected a Democrat to statewide office was more than a decade ago, in 2015.

But the race between incumbent Senator Cindy Hyde-Smith, a staunch ally of President Donald Trump, and her Democratic challenger, Scott Colom, proved tighter than expected.

The contest remains too close to call, with both candidates holding about 49 percent of the vote, according to The Associated Press (AP) news agency. Ninety-six percent of the ballots have been counted so far.

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As of Wednesday night, Colom was leading Hyde-Smith by a razor-thin margin of 1,138 votes, with a few thousand provisional and mail-in absentee ballots left to be counted.

Provisional ballots, also called “affidavit ballots”, are supplied to voters whose eligibility is uncertain. This can happen when a voter fails to present the appropriate identification documents at their local polling precinct.

Mississippi allows 10 days for such votes to be processed. Mail-in ballots, meanwhile, have a window of five business days to be received after the election, according to state officials.

In a statement released on Wednesday, Mississippi Secretary of State Michael Watson counted roughly 3,600 mail-in ballots still to be tallied, along with an “unknown” number of affidavit ballots.

“Counties have 10 days from the date of the election to submit their certified results to our office, which is November 13,” Watson wrote.

In the event of a deadlock, Mississippi is the only state in the country with no formal provision that would automatically authorise a recount. The candidates would need a court order to request one.

When will we know Alaska’s results?

In the northwestern state of Alaska, the Senate race could take days or weeks to be called, owing to the state’s vast rural distances and its ranked-choice voting (RCV) system.

Democrat Mary Peltola leads incumbent Republican Dan Sullivan by a meagre 637 votes as of Wednesday night, according to AP.

The race has become one of the most closely watched in the country. Nearly a quarter of the votes remain uncounted.

Ranked-choice ballots allow Alaskan voters to number the candidates in order of preference.

But the system can result in multiple vote counts. The initial count tallies the candidates ranked as voters’ top choice. If one of those candidates gets more than 50 percent of the vote, they win the election outright.

If no candidate passes the 50 percent threshold, the candidate with the fewest first-choice votes is eliminated, and a new round of vote-tallying commences. Supporters of the eliminated candidates see their votes redistributed to their second choice.

This process of elimination and recounting continues until one candidate ultimately wins a majority.

Alaska is one of only two states in the US to use ranked-choice voting for statewide elections, and critics blame the system for the slow vote tally.

According to Alaska’s Division of Elections, ranked-choice results will not be published until 15 days after the election, on November 18.

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The state is also under a microscope due to a controversy in the Senate race. Two candidates named Dan Sullivan competed in the contest.

One, Dan S Sullivan, is the state’s incumbent senator and an ally of President Trump.

The other, Dan J Sullivan Jr, is a retired schoolteacher widely rumoured to have been a Democratic plant to draw votes away from the incumbent.

Trump’s Department of Justice filed a lawsuit to prevent Alaska from certifying its election results, claiming the second Sullivan amounted to an attempt to pervert voter choice.

But a federal judge dismissed the suit on Sunday. The Justice Department has nevertheless pledged to appeal, a prospect all the more likely if Peltola prevails in the vote count.