Sen. Susan Collins (R-Maine) (Photo by Melina Mara/The Washington Post)

Sen. Susan Collins (R-Maine) said Sunday that she is open to potential Republican primary challengers to President Trump while also declining to endorse the president’s 2020 reelection bid.

Collins, who helped defeat Republican attempts to repeal the Affordable Care Act through Congress in 2017, argued that primaries help shape policy by allowing “a lot of viewpoints to surface.” Several other Republican senators have endorsed Trump’s 2020 aspirations, including Majority Leader Mitch McConnell (R-Ky.), Lindsey O. Graham (R-S.C.), and Sen. Ted Cruz (R-Tex.).

“I see nothing wrong with challengers — that is part of our democratic system,” Collins said on CNN’s “State of the Union.” “It’s healthy for our democracy.”

Collins’s comments come amid wide speculation about a Republican primary bid against Trump, who remains popular among Republican voters. Ohio Gov. John Kasich (R), who was defeated easily by Trump in 2016, has encouraged speculation he will challenge the president in 2020, traveling to New Hampshire this fall. Collins supported Jeb Bush and then Kasich in the 2016 election, and said in August 2016 that she would not be voting for Trump in that election.

Sen. Jeff Flake (R-Ariz.) has also been floated as a potential candidate to oppose Trump in a primary, although the senator said in November, “I don’t think that will be me. I think there are better candidates out there.”

This month, Trump brushed off the danger of a potential primary challenge, encouraging Kasich and Flake to get into the race. The last two presidents to serve only one term — Republican George H.W. Bush and Democrat Jimmy Carter — both faced primary challengers.

“I hope so,” Trump told Fox News when asked about the prospect of Kasich or Flake running.

About 89 percent of Republican voters approve of Trump’s job performance, according to the latest Gallup polling, roughly unchanged from his inauguration.

Collins recognized that a potential Republican primary challenger would be unlikely to beat Trump: “They would have an uphill climb."

Collins has voted with Trump 77 percent of the time, according to FiveThirtyEight’s vote tracker, including for Trump’s Supreme Court pick Brett M. Kavanaugh.