Given that continued rise in the stock market, it’s no wonder that Mr. Trump wants to take credit for it. Investor and traders say that the gains reflect hopes that Mr. Trump will enact business-friendly policies like lower corporate taxes, looser regulations and infrastructure spending. As many are quick to point out, if Mr. Trump takes credit for the stock market rally, he also must take credit for any reversal. And many are worried that market is getting ahead of itself as valuations rise even without full clarity on the administration’s policy priorities, setting stocks up for a resurgence in...