Story highlights Donald Trump won the Republican primary in New York on Tuesday

David Gergen: Psychologically, the tide could now reverse in Trump's direction

David Gergen is a senior political analyst for CNN and has been a White House adviser to four presidents. A graduate of Harvard Law School, he is a professor of public service and co-director of the Center for Public Leadership at the Harvard Kennedy School. Follow him on Twitter: @david_gergen. The opinions expressed in this commentary are his.

(CNN) Two weeks ago, after his decisive win in Wisconsin, it appeared that the odds for winning the GOP presidential nomination had suddenly and unexpectedly shifted in favor of Sen. Ted Cruz.

Donald Trump was then on the ropes, as he had failed to assure voters about fights breaking out at his rallies, engaged in needless attacks on Heidi Cruz, and then -- outrageously -- said that women who obtained illegal abortions should be criminally punished . Rarely have we seen so much self-destruction by a front-runner, so much so that one wondered whether Trump had a death wish.

Coming off his primary victory over Trump in Wisconsin, Cruz had an opening to put away The Donald, perhaps forever. And since then, Cruz has played the inside game extraordinarily well, winning a string of caucuses and peeling away a number of avowed Trump delegates who swore they would support Cruz on a second ballot.

But as we have seen in this most peculiar of all peculiar campaigns, two weeks can change the world. And so they have, climaxing in Trump's thumping victory in New York on Tuesday night

It's too much to say that the odds now favor either Trump or Cruz -- this Republican race could go in several different directions -- but there is no doubt that Trump has come off the ropes, revived his campaign and poses a formidable threat to anyone who tries to stop him from the nomination.

Read More