WASHINGTON — A partial shutdown of the federal government, which began early Saturday, is unlikely to make much of an immediate dent in the growth of the United States economy, but it could make life more difficult for millions of Americans in ways both big and small.

The shutdown could also contribute to the growing sense of worry on Wall Street, in part by raising fears about the ability of Congress to manage a more consequential deadline: the need to authorize an increase in government borrowing before the federal debt reaches the current limit, most likely in March.

President Trump appeared unfazed by an economic threat his critics contend is of his own making and that he initially said he would “own.”

“We are totally prepared for a very long shut down,” Mr. Trump said on Friday, as he continued to insist he would veto any government funding bill that did not include money to build a wall along the border with Mexico.