Assuming you were to put your foot on the scale a bit with the polling in order to demoralize the opposition, the only way you would be able to use that trick m

Assuming you were to put your foot on the scale a bit with the polling in order to demoralize the opposition, the only way you would be able to use that trick more than once would be to make sure to make sure that you quit doing it in the final days or weeks before the election. If the polling is far off from the results, you either get unwanted investigations or you lose credibility for future polling. Anyone with sufficient resources to influence something on this scale is likely playing a long game, and it's clear that the game is being played on some level. Look at the way the DNC started out with the bulk of the superdelegates for Hillary in order to give her an early credibility jump over the opposition, just in time to get actual credibility by winning Texas. Especially when someone is donating to a campaign, they want to donate to a winner.