1000 percent is a catchphrase in common use in the U.S. in the mid-20th century meaning highly enthusiastic support. For example, novelist Truman Capote wrote, "Prison is where she belongs. And my husband agrees one thousand percent." Playwright Loring Mandel wrote, "Those coal people, those pinball people. I want them behind us a thousand percent."
It was most famously used by Democratic Presidential candidate George McGovern in 1972. It backfired badly and became a byword for foolish and insincere exaggeration, and today is often used in irony or sarcasm.
On July 25, 1972, just over two weeks after the 1972 Democratic Convention, McGovern's running mate, Thomas Eagleton, admitted the truth to news reports that he had received electroshock therapy for clinical depression during the 1960s. Eagleton had kept his secret from McGovern. However McGovern had been running an emotional crusade against incumbent President Richard Nixon; Nixon supporters had counterattacked by suggesting that McGovern was crazy. The new evidence that his running mate had secretly undergone psychiatric treatment three times for mental illness destroyed the McGovern strategy. Eagleton was hospitalized in 1960 for four weeks for "exhaustion and fatigue." He was hospitalized for four days at the Mayo Clinic in 1964, and for three weeks in 1966. He twice underwent electroshock therapy for depression. Influential Democrats questioned Eagleton's ability to handle the office of Vice President, and McGovern's competence in choosing top officials. In response to intense pressure from the media and party leaders that Eagleton be replaced, McGovern announced that he was "1000 percent behind Tom Eagleton, and I have no intention of dropping him from the ticket."