Sen. Susan Collins, R-Maine, said Sunday she welcomes potential Republican primary challengers to President Donald Trump while also declining to endorse the president’s 2020 re-election 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 and elected officials have already endorsed Trump’s 2020 aspirations, including Majority Leader Mitch McConnell, R-Kentucky; Sen.