Final answer:
George W. Bush is the president who had a 90% approval rating at one point, which declined to 30% by the end of his second term.
Step-by-step explanation:
The president described as having an approval rating at 90% initially, but experiencing a decline to 30% by the end of his second term is George W. Bush. This steep decline is considered one of the most dramatic on record. After the September 11 attacks, President Bush's approval soared to 90%, reflecting a rally-around-the-flag effect; however, as his second term progressed, particularly with the unfolding challenges of the Iraq war and domestic issues, his approval ratings dwindled, ultimately reaching approximately 30%.