(CNN) Acting Defense Secretary Patrick Shanahan may be about to have the single most important day of his Pentagon career when he sits down before the Senate Armed Services Committee Thursday morning for televised hearing on the $700-billion plus defense budget.

His performance at the hearing may determine whether President Donald Trump finally nominates him to become the permanent Pentagon chief, according to several defense and administration officials that have spoken to CNN.

On Wednesday, the President was asked again by the White House press corps about Shanahan. "He is doing a great job. Shanahan is doing a fantastic job as secretary--as the acting secretary," Trump said. But when asked if he would nominate him to the job permanently, Trump still demurred. "Well I'm just telling you he's doing a great job look at what we've done with respect to the caliphate in Syria I was told by our previous person that it was going to take 2 years to knock it out and I did it in 3 weeks once we started."

The slam was apparently aimed at former Defense Secretary James Mattis, who resigned late last year in protest after the President ordered a sudden withdrawal of all US troops from Syria. There is no indication that the goal of even defeating ISIS's physical caliphate has been achieved in three weeks. In fact, US military officials warn that even after US backed fighters take back the last ISIS enclave, fighting will continue as committed ISIS fighters switch to becoming an insurgent attack force.

But it is the way Shanahan handles other key issues before the committee, especially Pentagon involvement in reprogramming defense funding for possible border barrier activity, that may be a determining factor in his relationship with the Senate Committee which would have to confirm his nomination.

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