WASHINGTON -- John McCain, the six-term Arizona senator and 2008 Republican presidential nominee, died Saturday at the age of 81, more than a year after he was diagnosed with glioblastoma , an aggressive form of brain cancer.The son and grandson of Navy admirals, McCain was a former Navy pilot and was held as a prisoner of war in Vietnam for more than five years. He was elected to Congress in the early 1980s and elected to the Senate in 1986, replacing retiring Barry Goldwater. McCain gained a reputation as a lawmaker who was willing to stick to his convictions rather than go along with party leaders, a streak that drew a mix of respect and ire.McCain underwent surgery in July 2017 to remove a blood clot in his brain after being diagnosed with an aggressive tumor called a glioblastoma, the same type of tumor that killed Sen. Edward M. Kennedy at age 77 in 2009.McCain rebounded quickly, however, returning to Washington and entering the Senate in late July to a standing ovation from his colleagues. In a dramatic turn, he later cast a deciding vote against the Republican health care bill, earning the wrath of Trump, who frequently cites McCain's vote at campaign events.McCain's condition worsened last fall and he had been in Arizona since December. McCain was a long-term survivor of melanoma, a deadly skin cancer. Doctors classified his brain cancer as a "primary tumor," meaning it wasn't related to his former malignancies.McCain ran unsuccessfully for the Republican presidential nomination in 2000, then won it in 2008 before losing the general election to then-Sen. Barack Obama.He returned to the Senate, determined not to be defined by a failed presidential campaign in which his reputation as a maverick had faded.When Republicans took control of the Senate in 2015, McCain, the scion in a decorated military family, embraced his new influence as chairman of the Senate Armed Services Committee, pushing for aggressive U.S. military intervention overseas and eager to contribute to "defeating the forces of radical Islam that want to destroy America."Asked how he wanted to be remembered, McCain said simply: "That I made a major contribution to the defense of the nation."He was a frequent target of criticism from President Donald Trump, especially for his vote against a Republican replacement for "Obamacare," the health care law approved under President Barack Obama.Trump signed a military policy bill in August named for McCain, but in a sign of their testy relationship, the president made no mention of McCain's name in remarks at a signing ceremony.The senator had been away from the Capitol since December. Republican Arizona Gov. Doug Ducey will name a replacement to serve out the remainder of McCain's term - through the 2020 election. The GOP currently holds a bare 51-49 Senate majority.McCain was married twice, most recently to Cindy McCain in 1980. He had seven children (both adopted and biological) from his two marriages, including daughter Meghan McCain, a prominent political commentator and co-host of "The View." He is also survived by his 106-year-old mother, Roberta McCain.