Oddly enough, it may wind up being the economy. Think of it this way: despite an economy that has been, at least in terms of conventional macro-indicators, quite strong, Trump has never been very popular. Indeed, his job approval rating has consistently run far behind where it should be given the state of the economy. Ron Brownstein notes:
"[P]olling throughout Trump’s presidency has consistently shown that economic improvement hasn’t lifted him as much as earlier presidents. Across many of the key groups in the electorate, from young people to white college graduates, Trump’s job-approval rating consistently runs at least 25 points below the share of voters who hold positive views about either the national economy or their personal financial situation.
The result is that Trump attracts much less support than his predecessors did—in terms of approval rating and potential support for reelection—among voters who say they are satisfied with the economy."
Brownstein adduces a lot more data along these lines but that's the gist of it: Trump is radically underperforming "his" economic indicators. Because of this, he is quite vulnerable,even now, for re-election despite an economy that continues to chug along.
But what if it stops chugging? Then perhaps the key positive that has been propping up Trump will disappear. As Ben White notes in Politico:
"Signs of a slowdown are mounting with weaker job growth, reduced manufacturing activity and a nervous Federal Reserve hinting at slashing interest rates — suggesting that Trump could suffer from terrible economic timing.
All recent presidents other than Bill Clinton experienced slowdowns in their first terms, but most sought reelection as the economy was improving. No president since Calvin Coolidge in 1924 has held on to the White House during an election year marred by recession.
Trump may avoid running for reelection in an official recession, especially if he gets his much desired rate cuts from the Fed. But there is a good chance he’ll be seeking a second term with the economy slowing and unemployment rising, especially if he continues to engage in bruising trade battles.
Voters tend to lock in their assessment of a president’s performance on the economy a few months before Election Day, meaning Trump’s strongest argument for four more years may not wind up being all that strong."
In such a case an already-vulnerable incumbent President will wind up being even more vulnerable. It's would be a nightmare scenario for Trump, with his greatest strength turning into a serious, and perhaps fatal, weakness.