Trump’s current approval rating is pretty similar to the approval rating for both Obama and Reagan at this point in their respective presidencies. (Trump 43%, Obama 45%, Reagan 43%.)
Reagan’s recovered sharply over the course of the election year, and he was comfortably over 50% when he faced Mondale.
Obama didn’t really recover, but he managed to pump his approval rating up to about 47% during much of the campaign, and squeezed it up to 49% during the last days of his re-election campaign, which got him narrowly re-elected.
(Obama’s approval promptly slumped again after his second inauguration, and he remained pretty unpopular until his lame-duck period suddenly made him the most popular politician in America.)
Most presidents see modest gains in their approval ratings during an election, probably because (partly) they’re no longer being compared to a non-existent ideal but to the actual flawed human opponent and (partly) all that presidential campaign advertising pays off marginally.
George H.W. Bush and Jimmy Carter are the big exceptions. H.W. was hit by an election year recession for which he was (rather unfairly) blamed. Carter never recovered from the failure of Operation Eagle Claw, the military attempt to rescue the Iranian hostages, and his subsequent address to the nation.
That recession in the Bush I Administration is something Trump should watch out for.
Another thing Trump should watch out for: both Obama and Reagan started their presidencies well above 50%, proving it was theoretically possible for them to win majority approval. President Trump’s best approval rating came on his very first day in office… when it was only 45%. Trump may have a much harder time reaching 49% than Obama or Reagan, because that would be all-new territory for him.