Washington (CNN) Republican Sen. Susan Collins of Maine said Sunday that she sees "nothing wrong" with 2020 primary challengers to President Donald Trump, adding that it's "healthy" for democracy.

"It's always interesting when we have primaries because a lot of times it allows different viewpoints to surface. It can help influence public policy down the road and it's healthy for our democracy," Collins said Sunday on CNN's "State of the Union."

Retiring Republican Sen. Jeff Flake of Arizona and outgoing GOP Ohio Gov. John Kasich have not ruled out a 2020 primary challenge to Trump.

Collins said it's "up to those individuals to decide whether or not they would oppose the President" and predicted that they would "probably have an uphill climb" against Trump.

The Maine senator was coy about whether she would endorse Trump's bid for a second term.

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