When planning an election campaign, you should base your message on your strategy. In other words, the message is something that doesn't change daily, but is fairly constant. If you take a closer look at great campaigns, message discipline is almost always a key element. In the case of Barack Obama's first presidential campaign, for example, it is stunning just how little his message changed over the course of more than a year. From the announcement speech, to the acceptance speech at the Democratic convention until the victory speech on election night, the key elements were the same. Another great example for message discipline, with a very different political ideology, was George W. Bush, who in some speeches even referred to himself in third person as the messenger.
This being said, I sometimes get asked in workshops when one should change the message. It's a tricky question and definitely a bad sign if there's a need for that during a campaign. The answer is as follows: You should change your message when the underlying assumptions, on which your strategy is based, change.
Let me give you an example for what I mean: Donald Trump's reelection campaign and the issue of the coronavirus. Trump had prepared to run for reelection with a booming economy at his back. He would make this the rationale for his reelection bid, a classic "are you better off now than four years ago" campaign. Then the coronavirus happened. Trump decided to stick to his strategy and basically ignored the issue. But it got bigger and bigger. By the time of the Republican convention, the campaign had become sort of an alternative universe: here the news with record number of cases, deaths, lock down and economic downturn, there the Republican convention where the main issue was barely mentioned. I personally think that the time when Trump got the coronavirus himself was the last moment where he could have – and should have – changed his message. Specifically when he was released from the hospital and had the attention of the entire world. He didn't. The rest is history.