WASHINGTON — For the first time since health secretary Robert F. Kennedy Jr.’s confirmation in February, visible fractures emerged in Senate Republicans’ united front of support for him. At the heart of the conflict: His stance on vaccines, which some Republicans had determinedly looked past before.

At a contentious Senate Finance hearing on Thursday, three Senate Republicans joined their Democratic colleagues in poking at Kennedy’s contradictory statements on mRNA vaccines and Operation Warp Speed, the initiative by President Trump that got Covid-19 shots to millions of Americans.

Their line of questioning highlights a conundrum for the health secretary: How can he cast doubt on the safety of Covid-19 vaccines, and vaccines overall, without criticizing a victory from President Trump

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