Sen. Lindsey Graham has still yet to congratulate President-elect Joe Biden following his election victory over President Donald Trump and is instead continuing to seek further legal challenges over disputed claims of alleged mass voter fraud.

In the hours after several news outlets including Fox News and the Associated Press had declared that Biden had won the election, Graham issued a statement stating he had received an affidavit from a postal worker in the key state of Pennsylvania claiming post office supervisors had a plan to "backdate ballots mailed after the election."

Graham said that he will be calling on the Department of Justice and the U.S. Postal Service to investigate these claims after receiving the allegations from Richard Hopkins, a postal worker in Erie, Pennsylvania.

Trump has repeatedly claimed that Democrats have "stolen" the election and that there has been widespread voter fraud, despite not offering any proof. In the past, substantiated examples of voter fraud have been found to have been minuscule.

Mail-in vote counting in Pennsylvania, a state which did not start counting until Election Day and has a three-stage system of counting to ensure accuracy and combat fraud, became a key issue this year as millions more votes were cast through the post because of COVID-19.

"It is imperative that all credible allegations of voting irregularities and misconduct be investigated to ensure the integrity of the 2020 elections," Graham said.

"The presidential election remains close in multiple states, and as Chairman of the Senate Judiciary Committee, all credible allegations of voting irregularities and misconduct will be taken seriously. I will not allow credible allegations of voting irregularities or misconduct to be swept under the rug.

"The expanded use of mail-in voting is making the post office the administrator of elections, not the local election officials. This change in mission requires there to be further scrutiny to ensure the system is beyond reproach.

"Every American should want our election processes to work accurately, and given the recency of such a large volume of mail-in voting, that will require oversight. Election outcomes are not determined by media outlets but certified, accurate vote counts.

"Officials in Pennsylvania should take allegations from Mr. Hopkins and others seriously before certifying a final outcome."

Graham is one of a number of Republicans who have not congratulated Biden on his win, including Senate Majority Leader Mitch McConnell. According to The Washington Post, McConnell plans to say little in public about Biden's win until he sees how serious some of the legal challenges against his victory are.

"Here's how this must work in our great country: Every legal vote should be counted. Any illegally-submitted ballots must not. All sides must get to observe the process. And the courts are here to apply the laws & resolve disputes," McConnell tweeted on Friday.

"That's how Americans' votes decide the result."

Newsweek has contacted the U.S. Postal Service for comment.

Incumbent candidate Sen. Lindsey Graham (R-SC) celebrates a win during his election night party on November 3, in Columbia, South Carolina.Sean Rayford/Getty