He was promised all he asked and seemed to have not only the confidence of the President, but his sincere good wishes.

The promises were all broken.

Only about half the troops were furnished that had been pledged, other war material was withheld and Scott had scarcely started for Mexico before the President undertook to supersede him by the appointment of Senator Thomas H.
Benton as lieutenant-general.

This being refused by Congress, the President asked legislative authority to place a junior over a senior of the same grade, with the view of appointing Benton to the rank of major-general and then placing him in command of the army, but Congress failed to accede to this proposition as well, and Scott remained in command: but every general appointed to serve under him was politically opposed to the chief, and several were personally hostile.
General Scott reached Brazos Santiago or Point Isabel, at the mouth of the Rio Grande, late in December, 1846, and proceeded at once up the river to Camargo, where he had written General Taylor to meet him.
Taylor, however, had gone to, or towards Tampico, for the purpose of establishing a post there.

He had started on this march before he was aware of General Scott being in the country.