"Mail-in voting has been used in a lot of places for a long time," Senate Majority Whip John Thune, the No. 2 Senate Republican, said in the Capitol. "And honestly, we got a lot of folks, as you know, who are investing heavily to try to win that war, it's always a war too for mail-in ballots. Both sides compete and it's always an area where I think our side -- at least in my experience -- has done pretty well."

The South Dakota Republican added, "I don't want to discourage -- I think we want to assure people it's going to work. It's secure and if they vote that way, it's going to count."

The comments come as a range of Republican officials throughout the country have reacted with growing alarm to the President's attacks on mail-in ballots, saying his unsubstantiated claims of mass voting fraud are already corroding the views of GOP voters, who may ultimately choose not to vote at all if they can't make it to the polls come November.

Amid the ongoing coronavirus pandemic, which has the potential to worsen in the fall, voting by mail is becoming an increasingly popular option since many voters may prefer not to wait in long lines at polling stations. Democrats could be handed a major advantage if their voters send their ballots by mail while Republican voters forgo that option because they are listening to the concerns of the President.

Asked by CNN if he's worried Trump will depress the GOP vote if he continues to make the claims, Thune said: "I think it's been expressed to him already in some states -- and I hope that message has been well received."

In recent days, Trump has continued to draw a misleading distinction between absentee voting and mail-in-voting -- which experts say are essentially the same -- and has repeated unsubstantiated claims about voter fraud, despite the fact that there is no widespread fraud in US elections.

The President repeated those claims last week while suggesting that the date of the election should be delayed -- something that the President does not have the authority to do and an idea that was met with swift pushback from congressional Republicans.

"With Universal Mail-In Voting (not Absentee Voting, which is good), 2020 will be the most INACCURATE & FRAUDULENT Election in history. It will be a great embarrassment to the USA. Delay the Election until people can properly, securely and safely vote???" Trump tweeted.

This week, the President has been training his attention on the battleground states of Florida and Nevada, urging voters in Florida, a state he won in 2016, to vote by mail, while arguing that Nevada, a state Hillary Clinton won in 2016, is unprepared for mail-in-voting.

"Nevada has ZERO infrastructure for Mail-In Voting. It will be a corrupt disaster if not ended by the Courts. It will take months, or years, to figure out. Florida has built a great infrastructure, over years, with two great Republican Governors. Florida, send in your Ballots!," Trump tweeted on Wednesday.