Former Oklahoma Sen. Tom Coburn, whose inflexible support of conservative policies placed him at the heart of many major congressional battles, has died at the age of 72. His former communications director, John Hart, confirmed that Coburn died Saturday morning "after a long battle with prostate cancer."

Coburn's former colleague, fellow Republican Sen. James Lankford of Oklahoma, remembered the senator as a "friend and a leader."

The state "has lost a tremendous leader," Lankford tweeted, "and I lost a great friend today. Dr. Coburn was an inspiration to many in our state and our nation. He was unwavering in his conservative values, but he had deep & meaningful friendships with people from all political & personal backgrounds."

Coburn served his state in Congress for two decades — first as a representative in the House, then, after a three-year spell out of office, as a senator during the administrations of George W. Bush and Barack Obama. Coburn rarely hesitated to spar with the latter over policy, despite his close personal friendship with the Democrat.

By the time he announced his early departure from the Senate in 2014, citing a fifth bout with cancer, the obstetrician-turned-congressional fiscal hawk had earned a reputation for obstinacy in pursuit of a slimmer federal budget — and a fitting nickname to go along with it: "Dr. No."

Perhaps few of his works were more emblematic of his time in Congress than his annual "Wastebook," a regular compilation of examples of what he deemed to be flagrantly unnecessary government spending.

His obstinacy could be felt on both sides of the aisle too, as in a 2008 op-ed he wrote taking his GOP colleagues to task for suffering from "paralysis and denial."

"Regaining our brand is not about messaging. It's about action. It's about courage. It's about priorities," he wrote at the time.

"Most of all, it's about being willing to give up our political careers so our grandkids don't have to grow up in a debtor's prison, or a world in which other nations can tell a weakened and bankrupt America where we can and can't defend liberty, pursue terrorists, or show compassion."

Vice President Mike Pence mourned Coburn in a tweet Saturday.

"Senator Tom Coburn was a great conservative voice in the United States Congress and American physician whose legacy will live on," Pence said. "Karen and I send our deepest sympathies and prayers to his family during this tough time."

Coburn is survived by his wife Carolyn and daughters Callie, Katie and Sarah.