CNN has an excellent rundown of then and now for Republican Senators up for reelection. (Note: Some of the quotations here expand upon what is detailed in the CNN article, sourcing linked.)

Thom Tillis 2014: “It’s just another example of where this President and [Then-incumbent late NC Dem Senator Kay] Hagan just have not worked on a comprehensive strategy”
Thom Tillis 2020: “When you’re in the middle of a crisis like this, I don’t think it’s productive to criticize everybody. It’s a very difficult job and lives are on the line.”

Susan Collins 2014: “The President has always been one step behind.”[1]
Susan Collins 2020: “I think it’s been mixed.”

Cory Gardner 2014: “Perhaps the CDC should quit spending money on things like Jazzercise, urban gardening and massage therapy and direct that money to where it’s appropriate in protecting the health of the American people”
Cory Gardner 2020: “I’m not going to play political pundit for you”

John Cornyn 2014: “The White House has done little over the past few weeks to inspire the confidence of Texans”[2]
John Cornyn 2020: “I don’t know any country in the world that has the Covid crisis under control — including the United States”

Joni Ernst 2014: “With Ebola, we see the President is very hands-off. He’s not leading.”[3]
Joni Ernst 2020: “I think that the President is stepping forward”

David Perdue 2014: “We have a commander in chief who has failed to take responsibility”[4]
David Perdue 2020: “It’s a totally different situation”