From NPR:

“The president also faces some significant headwinds for re-election in 2020. Just 30 percent of registered voters said they will definitely vote for Trump in 2020, while 57 percent said they will definitely vote against him.
....
For context, in 2010, when asked about then-President Barack Obama, just 36 percent said they would definitely vote for him, while 48 percent said they would not. Obama went on to win with 51 percent of the vote.”  

President Trump's national polling results, like Obama's in 2010, have little or no significance for two reasons. First, the time between now and November 2020 is far too long for current polling to have predictive value (see Obama situation in 2010, which was a function of the disappointing economic recovery). Second, national popularity does not predict the likely outcome in the handful of states that will determine the presidential election. 

In 2010 President Obama correctly believed that he was in pretty good shape in the swing states. His eventual opponent, Mitt Romney, was surprised to discover that in Pennsylvania, Ohio, Michigan, Wisconsin, Iowa and Minnesota he faced a “blue wall” constructed by the electoral college system.

The incumbent only won the popular vote by 51% vs. 47% for Romney, but Obama got 332 electors to Romney’s 206, which is 62% to 38%.

The effect of the anti-democratic electoral system was to magnify grossly the Obama margin. The professionals in both parties drew wrong lessons from this outcome.  

Republicans failed to grasp in 2012 that the electoral college system hurt their chances to win. When the system flipped in 2016 to give Republicans a big win, they did not realize that the system arbitrarily perverts the will of the people nationally. There’s no telling whether it will help or hurt a major party’s candidate. Their party would be far better off building a big national base and depending on Republicans everywhere to give their candidate a plurality or even a majority of the popular vote.

Democrats concluded from the 2012 election that the Great Lakes states were solidly blue, and so Clinton was a favorite. In fact, the margins in those states, as shown on the map that follows, were narrow:

Accordingly, a Republican who appealed to the particular demographic composition of voters in the Great Lake states could win. Therefore, these states would determine who won in 2016. The Clinton campaign’s policy stances were popular with most people, but not so much in the swing states.   

Both parties, and their camp followers, pay attention to national polls because they are frequent and ubiquitous. They are not only irrelevant; they are also distracting. Nothing matters for an incumbent except governing so as to maintain a base in the swing states. Going into 2020, these are Florida, Pennsylvania, Michigan, Wisconsin, and New Hampshire.

Is the shut-down hurting President Trump or helping President Trump in those five states? That’s the only relevant question in forecasting the 2020 outcome. I bet the White House polling shows he is still the favorite in those states. Hence he is the favorite in the 2020 campaign.