Impeachment must start in the House, and the rules of the House are heavily tilted toward majority rule. So it would need a majority vote to even get started, and it would be referred to a committee with a Republican majority. If, somehow, the investigation done there is fair and a recommendation to impeach passes the committee, it still needs to pass in the House again.

But then, it goes to the Senate, where a 2/3 majority (67/100) is required to convicted. So now we have gone from needing a handful of Republican defectors on the House (including at least one committee member) to needing 22 of 53 Republican Senators to agree to convict. That is an extremely high bar. If you were a Republican who is on the fence about Trump, it is simply not worth openly defying him unless you were certain Republican Senators would convict.