Here are the key moments from the debate that brought Republican presidential candidates head-to-head in North Charleston, S.C. on Jan. 14. (Sarah Parnass/The Washington Post)

In their heated back-and-forth in Thursday's sixth Republican presidential debate, Sen. Marco Rubio (R-Fla.) threw out a lot of things that upset Sen. Ted Cruz (R-Tex.).

Rubio claimed Cruz supported legalization of undocumented immigrants and expanding green cards and guest worker visas. He also said Cruz doesn't vote for defense budgets. Or really any budgets.

"Every single time that there has been a defense bill in the Senate, three people team up to vote against it: Bernie Sanders, Rand Paul and Ted Cruz," Rubio said.

Cruz was visibly upset. "At least half of the things Marco said are flat-out false. They're absolutely false," he said, going on to refute Rubio's characterization of his immigration record and adding that he actually voted for one of Rubio's amendments to increase military spending.

"And as president, I will rebuild the military and keep this country safe," Cruz finished.

Arguing on the debate stage about defense spending bills and immigration amendments is par for the course for these two rival senators, who tend to get sucked into bickering over arcane procedure. Rubio’s attack about Cruz voting against defense spending bills would probably have had more impact on Capitol Hill than in South Carolina.

[The GOP debate proved being a senator is bad for running for president]

But Rubio’s line about what Cruz has actually voted for raises an interesting point. Cruz has devoted much of his Senate career to one thing: obstruction. And he's pretty good at it.

In the classic measure of what makes a senator successful  -- passing bills into laws -- Cruz is middle of the pack. He's not the least or the most active member of the Senate, and he has a fairly typical record for a first-termer.

According to Congress.gov, in his first two years in Congress, Cruz introduced 32 bills, resolutions and amendments -- they range from repealing President Obama's 2010 health care law to approving the Keystone XL pipeline. One bill the president actually signed into law in 2014: It allows the president to deny visas to "known terrorists" from serving as U.N. ambassadors, which is headquartered in the United States. It was aimed Iran's proposed ambassador to the United Nations, who was involved in the 1979 Iranian hostage crisis.

Cruz is stepping up the pace in the 114th Congress, which started in January 2015 and is halfway over. He's introduced 30 bills so far, and one that he championed was signed into law by Obama two months ago: a bill giving private companies rights to own what they mine on asteroids in space and to help push government-funded space travel more toward the private sector.

But on the campaign trail, Cruz doesn't brag about what he's done. He brags about what he's stopped. From The Washington Post's Katie Zezima and David Weigel:

“I will acknowledge that when I’m in the Senate dining room I’ve sometimes wondered if I need a food taster,” Cruz said to laughs in Fort Dodge, Iowa, last month. He added: “If you ain’t never stood up to Washington, at any time in your life, you’re not gonna suddenly discover the courage to do so if you happen to land at 1600 Pennsylvania Avenue.”

While that's a far more difficult thing to measure -- how can you prove something didn't happen because of you? -- we'd argue Cruz isn't just in the middle of the pack when it comes to obstruction. He's leading it.

In Washington and outside it, Cruz is best known for going to war with the GOP on everything from Obamacare and immigration to Planned Parenthood and the Export-Import Bank. Even though Cruz has never succeeded in actually blocking any of this, he has delayed them -- and more importantly, raised his national profile considerably in the process.

In December, I detailed this incident a year earlier, when Republican leaders in the Senate headed home for the weekend thinking a spending bill would be put to bed on Monday:

It was a Friday night, and the spending bill vote was scheduled for Monday. Senate leaders had already gone home; some were on trains and planes back to their districts for the weekend when Cruz took to the Senate floor and demanded lawmakers go on-record for or against Obama's executive actions to defer deportation of some 5 million undocumented immigrants.

His colleagues were not impressed.

Everyone had to turn back around. Cruz forced the Senate into a Saturday session. Senators on both sides argued Cruz ended up giving then-Senate Majority Leader Harry Reid (D-Nev.) more time to push through the president's judicial and executive nominees. The spending bill passed anyway.

"I don’t see an end goal other than just irritating a lot of people," said Sen. Orrin G. Hatch (R-Utah).

Hatch may have missed the point. Throwing a wrench in the Senate was exactly what Cruz wanted to do. Using the Senate's procedural tool box to try to block or halt something he doesn't like has been Cruz's MO since day one, said Molly Reynolds, a congressional analyst with the Brookings Institution.

"He's really displayed an apt for using the Senate's procedural tools," Reynolds said.

Cruz has also used his power of persuasion to try to stop things from going forward.

In a 2014 New Yorker piece, Jeffrey Toobin detailed how before Cruz even officially took office in the Senate, he tried to stop something from going forward.

Cruz came to Washington in 2012, about a month before being sworn in. It happened to be the day the chamber was debating a fairly uncontroversial United Nations treaty on the rights of people with disabilities. Over lunch with his future Republican colleagues, Cruz tried to persuade them to oppose it, saying it would interfere with America's sovereignty.

“I was a newly elected senator who hadn’t even been sworn in yet, but I did just pass on, having just come from the campaign trail, that issues of U.S. sovereignty resonate powerfully with the American people,” Cruz told Toobin.

The treaty failed -- even though former Republican majority leader Bob Dole (R-Kan.) had been rolled out in a wheelchair on the Senate floor to lobby for it, looking dejected.

Sen. John McCain (R-Ariz.), who to this day is no fan of Cruz, called it one of the most embarrassing moments of his Senate career. Cruz called it a win.

"I urged my soon-to-be colleagues to protect U.S. sovereignty," he told Toobin, "and ultimately they did so."

We should note that the firebrand Texas senator, who is tied or leading in polls in Iowa, hasn't pioneered this idea of using Senate procedure to block something. As Congress has become more deadlocked and less productive over the past few decades, senators have had to find increasingly creative ways to make their voices known and shut things down.

But we'd argue he's the best so far at it. And that's because what Cruz wants out of the Senate is entirely different, to the point of almost being antithetical, to the traditional definition of a senator.

Some lawmakers want to go to Washington to pass laws. Cruz's rival Rubio could reasonably be considered among them. Some go to Washington to try to stop things that their supporters don't want and to take political/principled stands at all turns.

Those two things aren't mutually exclusive, Reynolds noted. But since before he even got to the Senate, Cruz seems to have placed obstructing Washington over legislative productivity.

"I would say that if as a senator your goal really is to build a broader political reputation," Reynolds said, "than the kinds of things we've seen Cruz do in his first term shouldn't surprise anyone."

Which is why in the long run, Rubio's attack on Cruz's record may have actually helped out Cruz appeal to a segment of the GOP that would prefer politicians who vote no.