The colleague who objects before the slide is even up is not evaluating your proposal. They are reacting to something else entirely, and it rarely has much to do with the content.
What the research found
Solomon Asch's classic 1951 conformity experiments asked participants to judge something as simple as the length of a line, and found that a meaningful share changed their answer to match an incorrect group consensus, even when the correct answer was visually obvious to them. People do not evaluate information in isolation. They read it against what they believe others in the room already think, and adjust accordingly, often without realizing they have done so.
Why this matters before a proposal is presented
A proposal argued only on its own merits is competing with whatever the room already believes other credible people think of it. If the visible starting consensus is skeptical, the content is fighting uphill regardless of its quality. If a few credible people are already on record in support, the same content is received differently before anyone has actually evaluated it on its own terms.
Where to start
Before presenting something contentious, find one or two people the room already respects and get their view on record first, even informally.
This is not about circumventing scrutiny, the proposal still has to hold up. It is about recognizing that the room's starting point is rarely the content alone, regardless of how good the argument is.
Reading who the room actually respects, versus who just has seniority, is a judgment worth getting right before you decide whose support to seek first. What this gives you is the reason that groundwork matters as much as the content itself.
There is a specific way to ask someone for informal, early support without it feeling like you are lobbying them, and it is worth having ready before your next contentious proposal.