Majority Leader Mitch McConnell: "President Trump is 100% within his rights to look into allegations of irregularities and weigh his legal options".
But he did say he was authorising prosecutors to "pursue substantial allegations" of irregularities of voting and the counting of ballots.
The attorney general said investigations should be conducted only into credible allegations of irregularities that, if true, "could potentially impact the outcome of a federal election in an individual state".
The Epoch Times Editorial Board will not declare a victor for the 2020 presidential election until all results are certified and any legal challenges are resolved across the country.
The Electoral College is slated to formally confirm Joe Biden's victory on December 14 and the Democrat will be sworn into office late January.
The previous Justice Department policy, created to avoid interjecting the federal government into election campaigns, had discouraged overt investigations "until the election in question has been concluded, its results certified, and all recounts and election contests concluded".
It came two days after the projection of former Vice President and Democratic candidate, Joe Biden, as victor of the keenly-contested presidential election.
Democrats and the Biden campaign said Barr was fueling the very far-fetched claims he claimed he was guarding against.
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This comes as top Republicans are backing President Trump's refusal to concede the election, claiming - without providing any evidence - that the vote was marred by fraud.
In a tweet, Monday, Trump said that Pennsylvania had "prevented us from observing the vote count" and that there were invalid votes in Nevada. It was swiftly deleted, but not before people took notice, joking it was the same voter fraud Trump has been harping on (which is not fraud).
Trump's campaign on Monday filed a lawsuit to block Pennsylvania officials from certifying Biden's victory in the battleground state, where Biden led by more than 45,000 votes.
"You are the most senior leaders in the United States Department of Justice and I trust you to exercise great care and judgment in addressing allegations of voting and vote tabulation irregularities", Barr wrote.
The Epoch Times has yet to receive a response from the Department of Justice for comment.
"The process has not failed our country in more than 200 years, and it is not going to fail our country this year", said Republican senator Susan Collins, who won her re-election bid in ME and has congratulated Mr Biden on his victory. Will the U.S. Supreme Court really buy that there's some inherent unfairness in mail voting, which is allowed in many jurisdictions and was approved by a Republican legislature in Pennsylvania?
Of course Arkansas Attorney General Leslie Rutledge joined other Republicans in asking the U.S. Supreme Court to throw out millions of mail ballots in Pennsylvania.