candidates who will buck former President

Inside McConnell’s Campaign to Take Back the Senate and Thwart Trump
Senator Mitch McConnell and his allies are working furiously to recruit
Inside McConnell’s Campaign to Take Back the Senate and Thwart Trump
Senator Mitch McConnell and his allies are working furiously to recruit candidates who will buck former President Trump.
Source: NYTimes (on the home page at this moment)
Does "buck" here mean "resist"? Oxford dictionary says when "buck" means "resist", it resists something; it doesn't say "resist someone".

In that context I consider it to mean 'challenge'.

Not a teacher.

I agree with Yankee here, in the context of the article it seems to mean "to challenge", "to oppose".
Read through the article again, I think it should be pretty clear.