Aside from occasional jabs and preliminary A/B testing, Trump should preserve heavier attacks against the 2nd tier for late this year or early next because he’s going to be very happy if either of the most likely 2nd tier candidates, Warren and Harris, get the Dem nomination. No need to short circuit them before they get a chance to take the lead while Biden is either taking his current lead for granted or gotten lost in his closet, and as both Senators inch closer to overtaking Sanders for second place.
As the greatest phony in a primary field flush with them, Warren will make for an easy sparring partner in 2020.
Should Harris get the nomination, Trump can tie her to all sorts of coverups and scandals associated with the California Democratic Party that were buried by her when she was the state’s AG. Those scandals include “creative” accounting in the state budget, failures to investigate well-connected state energy companies such as PG&E for faulty electrical wiring, the state’s merciless anti-business policies, a state’s failed high speed rail system, rampant statewide homelessness, sexual assault scandals involving the state party and the state university system, extensive corruption by California government workers, and much more.
The time when Warren or Harris will start gaining real primary traction will come when one passes Sanders for second place. At that point, whoever is the new silver medalist will draw media hype & headlines about momentum. Then will come more campaign funding, and party endorsements needed to take out Biden.
It is around the time Sanders falls to third place that Trump can indirectly help Warren or Harris by painting Biden as an “establishment” Democrat who backed NAFTA and the Pacific Trade Deal (which Trump withdrew America from in 2017) against the interests of his blue collar Democratic supporters. While Trump’s jibes will superficially appear to be only attacks against Biden, they will indirectly help Warren or Harris because they will, in their attempt to take first place from Biden, also be attempting to frame him as a corporatist-insider Democrat, a moderate DLC throwback to Bill Clinton’s 1990s.
The more of an outsider-insider contrast Warren and Harris can draw with Biden the more likely Trump will get one of those ladies as his opponent in 2020.