When Trump tweetedthe tweet featured above Washington Post decided to fact check this. They noted that they did not know what poll he used but that they found the most recent poll to be at 49%. The went on to discredit the polling organization by claiming that the organization is skewed towards Republicans.
Many issues with the fact check arise rather quickly. The poll history from the daily presidential approval rating was at 50% that day. It is true that the organization leans Republican but this is only because the organization polls “likely voters” and Republicans are always more likely to vote historically.