Manchin, Murkowski for bipartisan support to reauthorize the Voting Rights Act. Bill needs 9 more GOP votes to pass in the Senate.

• Sens. Murkowski and Manchin called for bipartisan support for voting rights legislation on Monday.
• In a letter to Congressional leadership, the two moderates said "inaction is not an option."
• But the legislation faces an uphill climb in the 50-50 Senate without additional Republican support.

Two of the Senate's most notorious moderates are urging Congress to reauthorize the Voting Rights Act and attempting to bolster bipartisan support for the landmark legislation.

Sen. Joe Manchin of West Virginia, a Democrat, and Sen. Lisa Murkowski of Alaska, a Republican, wrote a letter Monday calling on Congressional leadership to advance a reauthorization of the voting rights access bill, saying they "must not allow" voting to become a partisan issue.

"Inaction is not an option. Congress must come together - just as we have done time and time again - to reaffirm our longstanding bipartisan commitment to free, accessible, and secure elections," they wrote.

The 1965 Voting Rights Act has been amended five times since it was enacted and each time saw large, bipartisan majorities. The bill was most recently reauthorized in 2006 when the Senate passed The Fannie Lou Hamer, Rosa Parks, and Coretta Scott King Voting Rights Act Reauthorization and Amendments Act in a 98-0 vote, without a single dissenting vote.

But in 2013, the Supreme Court struck down the formula established by Congress, which required state and local governments that sought to make voting and election changes to get federal clearance prior to any changes, effectively gutting the law.

The decision has proved relevant again in recent months as GOP-led states across the country have moved to pass restrictive voting laws.

Though Manchin and Murkowski's letter does not explicitly name the bill, a Manchin aide told NBC News that the duo is referring to the John Lewis Voting Rights Advancement Act, which aims to revive the preclearance formula and strengthen the original 1965 law.

Murkowski's backing is particularly notable as she's the only Republican Senator to have publicly shown support for the bill. She was an original co-sponsor when the bill was introduced in the last Congress.

But the legislation still faces an uphill climb in the 50-50 Senate, where Democrats will need to either find nine additional Republicans to join Murkowski or find a way to gut the filibuster - two scenarios that remain unlikely.

Manchin's support comes as he faces growing criticism from the left for being the only member of his party to oppose the "For The People Act," a broader voting rights bill that aims to overhaul elections. The Democratic-controlled House has already passed that legislation but has not yet taken a vote on the John Lewis bill.

Manchin has repeatedly said he wants bipartisan support behind any attempt to overhaul federal election laws.