WASHINGTON -- When I entered Sen. Tom Coburn's office on Capitol Hill, he declined my offer to shake hands, telling me he had a cold. That reminded me, I said, of the time when Sen. Ted Cruz gave me a note saying he had lost his voice due to sickness and couldn't answer questions.

"That's what happens when you speak too long on the floor," Coburn cracked -- an obvious reference to Cruz's 21-hour speech last September seeking to defund Obamacare. Coburn strongly criticized the strategy then, and he didn't smile now.

But the 65-year-old Oklahoma Republican usually doesn't. His gruff, intense presence and lack of artifice is startling in the corridors of Capitol Hill -- though not for much longer.

Coburn is exiting the Senate at the end of this year. First elected to the upper chamber in 2004, he had always said he would serve only two terms there. He is leaving two years early. (He also served three terms in the House in the 1990s.)

Before he goes, he had some complimentary words for Cruz, with whom he shares a strong dislike of Obamacare despite their difference on tactics. "We're not estranged at all," Coburn said of the Texas Republican. "You've got to learn how to operate in the Senate, and he has and he will. And he'll learn more. And I learned as I came into the Senate. But I think he's a positive influence."

While Cruz will keep fighting battles in the Senate, Coburn has decided to focus his energies elsewhere.

When he announced his upcoming retirement last month, it was reported that his decision was based on his being diagnosed with a recurrence of prostate cancer. Coburn insists that the disease -- he has already survived colon cancer and melanoma -- has nothing to do with his decision to leave. Reporters "didn't read my press release," he said, referring to the line that declared his departure "isn’t about my health, my prognosis or even my hopes and desires."

"It's time for me to go do something else," Coburn said. "I know me. I've made lots of shifts in my life, and I know when it's time. My faith comes into that. I pay a lot of attention to what I think I'm supposed to be doing. ... And it's just time for me to do something else. So I'm getting ready to walk through whatever door opens."

"I don't have any set plans whatsoever," he said.

There are two exceptions to that statement. He has plans to play golf, a game he loves and has rarely been able to enjoy during his time in Washington. And he is going to lend his support to a growing effort in state legislatures across the country to call a convention to amend the Constitution with the aim of limiting the size and reach of the federal government.

"I'm going to be involved with the Convention of States. I'm going to try to motivate so that that happens. I think that's the only answer," Coburn said. "I'm just going to go around and talk about why it's needed, and try to convince state legislatures to do it."

The Georgia state Senate on Tuesday became the latest legislative chamber to vote to call for a national convention. Similar efforts are underway in Wisconsin, Virginia, Alabama and a few other states. Mark Meckler, a former leader of the Tea Party Patriots and founder of Citizens for Self-Governance, the group behind the Convention of States Project, said that he expects 10 to 15 states to make "a serious effort" to pass similar legislation this year. At least two GOP governors who could be 2016 presidential hopefuls, Louisiana's Bobby Jindal and Ohio's John Kasich, have said they support the idea.

Under Article V of the Constitution, if two-thirds of state legislatures -- or 34 states -- call for it, Congress shall convene a national convention, to which the legislatures will send delegates. The convention may propose constitutional amendments, which will then need to be approved by three fourths of the states -- 38 in all -- through votes either in the legislature or at a state convention.

In recent years, state legislatures have passed measures supporting the idea of a constitutional amendment requiring the federal government to balance its budget, and Kasich in Ohio has actively backed such a move. But the effort mentioned by Coburn is organized not around a specific amendment, but rather a specific subject: "limiting the power and jurisdiction of the federal government."

It is an idea proposed recently by conservative radio talk show host Mark Levin and since picked up by former Fox News personality Glenn Beck. Around 100 state lawmakers from a reported 32 states met last November to discuss the idea.

Coburn's decision to make this a cause of his own is a symbolic shift. He has long railed against the institutional corruption of Washington, arguing that careerism in Congress and self-protection by lawmakers of both parties make the nation's capital immune to pursuing real reform and to making the tough choices and difficult compromises necessary to get results.

His decision to leave Congress now and to focus his energies on the national convention idea is a loud statement that he doesn't believe Washington can be changed from the inside.