WASHINGTON — Jeb Bush had a plan for the last six months he spent as an unannounced candidate for president. He would raise tens of millions of dollars, distinguish himself from his brother’s controversial presidency, start winning over conservatives and establish himself as the Republican to beat for his party’s nomination.

Other than raising the money, little has gone as he had hoped. He has been torn between defending and distancing himself from George W. Bush, been unable to assuage party activists uneasy with his immigration and education views, and run into a wall of opposition on the right. And now, as he prepares to make his candidacy official on Monday, Mr. Bush finds himself in a position he could not have imagined: Part of a pack of candidates, and the target of questions about his own competence and conservatism.

Frustrated, according to his associates, by elements of his political operation and performance so far, he appointed a new campaign manager last week who is preparing an aggressive new approach to the race. Yet Mr. Bush still faces fundamental challenges in appealing to a Republican primary electorate that is much different from the one his father or even his brother faced — a party no longer willing to automatically anoint the pragmatic, well-financed, establishment-aligned candidate that the Bush name personifies.

“He just hasn’t met the expectation level of what we expected of a Bush,” said Senator John McCain, Republican of Arizona, who is supporting Senator Lindsey Graham’s candidacy but likes Mr. Bush. “And that’s been a hindrance to him.”