Being "denied his party's nomination" is not how it works, brainiac.
Even the liberal controlled and edited wikipedia speaks to how it happened with Johnson. I see history repeating itself now:
"After the 1966 mid-term Congressional elections, his re-election bid in the 1968 United States presidential election collapsed as a result of turmoil within the Democratic Party related to opposition to the Vietnam War. He withdrew from the race amid growing opposition to his policy on the Vietnam War and a worse-than-expected showing in the New Hampshire primary.
Johnson's approval ratings stayed below 50%; by January 1967, the number of his strong supporters had plunged to 16%, from 25% four months before.
Johnson also blamed the press, saying they showed "complete irresponsibility and lie and misstate facts and have no one to be answerable to." He also blamed "the preachers, liberals and professors" who had turned against him.
By year's end, the Democratic governor of Missouri warned that Johnson would lose the state by 100,000 votes, despite a half-million margin in 1964. "Frustration over Vietnam; too much federal spending and... taxation; no great public support for your Great Society programs; and ... public disenchantment with the civil rights programs" had eroded the President's standing, the governor reported."
Move ahead to 2011: Frustration over mideast wars; too much federal spending and taxation; no great public support for stimulus programs and obamacare; and public disenchantment with continued record unemployment has eroded the president's standing. Hell, if the NY Times is publishing that DEMS are "fretting out loud" about his standing; something is clearly happening.