Deciding when to buy and sell based on market conditions is called "Timing the market". It's a 4-letter word in the broker business. We'll come back to that.
The President, via the Fed, absolutely is capable of pumping up the economy and creating a stock market "bubble". One incentive for a president to do this is an an approaching election, since the economy (pro or con) can be a major selling point for a candidate.
Why they cannot possibly recommend timing the market
The problem is, as I alluded, No broker on earth is going to advise you to "time the market". Think about it. If timing the market is a responsible thing to do, then a broker's job would include telling you to when to step in and out of the market, and the broker could be held financially responsible for bad advice.
How would this even work? Brokers aren't in the business of providing performance insurance to small time investors, so they don't want to be in the position of being held responsible for market ups and downs. Even if they got it right, it would incite panic, as no broker wants to be the one who didn't tell their clients to get out when all the other brokers are saying that. So "Timing the Market" is something brokers simply cannot abide. They can't do it. They can't tell you to time the market.
Brokers want you to view the market as a savings account, and put money in when you have it, and take money out when you need it.
There's a partial case to be made
The problem is, when you do time the market, that is not guaranteed. There is an element of risk. The market could continue going up after you sell out, in which case that's gains you didn't make. Or it could go down somewhat before you sell, costing you gains.
Or presumably, your goal is to step back into the market when it's at the bottom, i.e. Wall Street is having a 50% off sale. But again, how do you know when the bottom bottom is? You could get on too soon and watch it drop more. You could get on too late and have lost some gains.
I think AAPL peaked at $27 and I got out at $25 (8% loss). And it bottomed out at $11.80, and I got back in at $12.86, so 9% missed opportunity right there. So that's 17% I "ate" on both sides of the equation. But I nearly doubled my holding.
Of course I could've just stayed in, taken the beating, and my stock value would've recovered in a little over a year.
The upshot is, that timing the market is dangerous.
Is it unpredictable? Not so much. After all, go back to what stocks are and the basic principles of evaluating a company's worth. You see a lot of outliers - look at Tesla, it's worth more than GM. Are GM's fundamentals worth more than Tesla, who owns one assembly plant? Oh, you betcha. It is possible and commonplace for stock prices to deviate significantly from the business fundamentals supporting that price. So it's up to you to look at the fundamentals of stocks, versus their trade value, and evaluate whether the trade value justifies the fundamentals, and whether the stocks are overpriced or under-priced.
When timing the market, your goal is to do that for the whole market.
BUY low. SELL high.
In 2008, much like AAPL, the market recovered all of its lost value in a couple of years.
But what unfortunately happens with novice investors is, they see all the terrible news reports, and they go "The sky is falling! The market has forsaken us! Preserve what little value you have left! SELL SELL SELL!"
So, when a novice investor does that, here's the important thing. That is timing the market. Get it?
And it fails for these novices because they are violated the rules: "Buy low, sell high".
The correct thing to do, when stock brokers are jumping out of windows on Wall Street is, buy. Wall Street is having a half-off sale.