By Leah Douglas
WASHINGTON, Nov 6 (Reuters) - US Senate candidates in Mississippi are battling over the treatment of ballots still being counted in the race, one of two remaining Senate contests yet to be called after Tuesday's midterm elections.
The outcome in Mississippi and in Alaska, where the Senate race is also not yet called, will determine whether the Democratic Party extends its sweeping election victory in which it seized at least 54 of the Senate's 100 seats, and also took control of the US House of Representatives.
Democrat Scott Colom narrowly leads incumbent Republican Senator Cindy Hyde-Smith by 49.0% to 48.8%, according to vote counts compiled by Reuters and USA Today, which are partnering this year to collect live election results for Senate and House races.
Hyde-Smith sent a letter on Thursday to all 82 Mississippi county election commissioners demanding that their offices preserve all affidavit, or provisional, ballots and separate them from regular and absentee ballots.
"These measures are necessary to ensure that each affidavit ballot’s eligibility for counting and ultimate disposition can be verified," said the letter from law firm Lex Politica, which represents Hyde-Smith.
Colom sent a letter later in the day to the commissioners calling Hyde-Smith's request unlawful, in part because it could result in different treatment of ballots across counties.
"If you depart from ordinary election procedures as you have been asked to do, you will violate Mississippi voters’ rights and Mississippi election law," said the letter from Elias Law Group, which represents Colom.
Those affidavit ballots, which require extra review because of questions about voter eligibility, could determine the outcome of the race. Between 12,000 and 20,000 remain to be counted, Mississippi Secretary of State Michael Watson said on Thursday.
If Colom wins, he would be the first Black US senator elected from Mississippi since 1874.
Democrats' midterm victories were propelled by widespread discontent with President Donald Trump's policies, and the party will control Congress when the new session starts in January.
Mississippi counties have 10 days from Election Day, or until November 13, to submit certified election results.
It is the only US state without a legal provision for vote recounts in statewide elections. Candidates could pursue a legal challenge to exclude certain ballots, which could change the vote count.
(Reporting by Leah Douglas in Washington; Editing by Andy Sullivan and Jonathan Spicer )