There’s already a rhythm to the shutdown fight: House Republicans huddle in the Capitol’s basement to plot their next move. They emerge with a plan. Then Harry Reid hits the Senate floor and swiftly dismisses it in the harshest possible terms.

“It’s just another wacky idea from the Tea Party-driven Republicans,” Reid said Tuesday, knocking the House’s new plan to temporarily fund portions of the government. “You can tell that the Tea Party Republicans still want to keep the government shut down.”

That was of a piece with Reid’s jabs on Monday, when he shot down three “Tea Party anarchist” bills attacking Obamacare as well as the House’s final pre-shutdown demand to go to conference.

The message was the same every time: either send a bill that funds the government without taking potshots at the health care law, or go home.

For Tea Party Republicans like Sen. Ted Cruz, the current showdown is a brutal introduction to the cutthroat politics of Harry Reid–an unassuming man who plays politics to win, rules be damned.

Reid’s tactic in this battle has been to hold Democrats in line, and watch his foes turn on each other as they tank in the polls for resisting changes to Obamacare, which, ironically, rolled out the day the government shutdown.

Republicans shouldn’t underestimate him. Reid’s been a deal-maker as often as a dogfighter over the years, alternating roles as the need arises.

He negotiated a spending deal to avoid a government shutdown shortly after Republicans retook the House. Then, in the 2011 debt ceiling debate, he urged the White House to take a harder line against Republican demands only to rebuffed as Obama tried unsuccessfully to reach a “grand bargain” with Boehner.

This time, the House’s extreme demands and Democrats’ relative unity have freed him to poke at his opponents like never before.

In recent weeks, Reid has stuck closely to Sun Tzu’s old adage: ”If your opponent is of choleric temper, seek to irritate him.” So far, so good. ”Harry Reid has proven to be a skillful political hit man and name caller,” Congressman Tom Graves, a Georgia Republican, complained Monday in a statement. “He never misses a chance to inject venom into the legislative process.”

Reid’s floor speeches were nothing compared to the hurt in store for Boehner the next morning. After rallying Republicans against health care subsidies for members for Congress, the Speaker woke up Tuesday to a Politico story unveiling a massive trove of email exchanges, phone calls, and even a presidential meeting in which Boehner sought to preserve the same benefits. Reid’s chief of staff David Krone later admitted to leaking the documents and his office was more than happy to throw some acid on the wound after the fact.

“Senator Reid appreciates Speaker Boehner’s cooperation and tireless efforts to work through this difficult issue,” Reid spokesman Adam Jentleson told Politico in the piece.

For the numerous Republicans elected in 2010 and 2012, dealing with Reid may be a relatively new experience. Cruz, the most prominent leader of the defund Obamacare movement, has been his colleague for barely nine months. The two are in some ways mirror images: Reid is, despite his gift for insults, a soft-spoken orator who prefers to work behind the scenes. Cruz is happy to hold the floor for 21 hours at a time and espouses an outsider approach based on rallying grassroots pressure.

Conservative Republicans like Cruz are betting everything on the expectation that Reid will either cave himself on Obamacare or fail to hold his caucus together. If they’re wrong, they won’t be the first ones to blow it. Those who know Reid best warn Cruz and his allies that he’s a dangerous foe.

“I think he is a really easy guy to underestimate,” Nevada political columnist John Ralston told MSNBC. “The number of people who’ve underestimated Reid and suffered because of it is a long list.”

The prime example is Reid’s 2010 re-election. For most of the race, Reid was considered a major underdog. The Tea Party wave was crashing on his state hard and, as Senate leader, he was especially vulnerable to populist attacks. But in a scheme worthy of House of Cards’ Frank Underwood, Reid systematically undermined or elevated his potential opponents for years until he got the exact one he wanted: Sharron Angle.

More recently, Reid bullied Mitt Romney with ethically questionable but politically effective tactics, claiming he had anonymous sources who had told him Romney had not paid taxes for years. An irritated Romney said in an interview he had never paid less than 13%. When he released his tax estimates to prove Reid wrong, it turned out one year had fallen below his public claim–so Romney chose to forgo hundreds of thousands of dollars in (perfectly legal) deductions. All in all, Reid’s trolling cost him about half a million dollars.

Reid had a hand in plenty of knockdown fights in DC as well. In Obama’s first term, he shepherded his major accomplishments into law, including the ACA, despite a razor thin margin in the Senate. Most recently, Reid won a showdown with Republicans over Obama’s cabinet picks, which ended with GOP senators agreeing to approve many long-delayed appointees. He’s had his share of failures as well, namely the Senate’s effort this year to pass gun purchase background checks. But overall, he’s maintained a reputation as a difficult opponent with a knack for finding the right angle.

Jim Manley, Reid’s former longtime aide, said that his proudest memory of his old boss actually took place nearly a decade ago when Reid was Minority Leader. After the 2004 election, President George W. Bush called on Congress to work with him on a plan to privatize Social Security. Reid immediately whipped members against the idea, then worked to pressure Republicans to keep it from ever seeing a vote.

“He believed accurately that this tack was nothing more than a fig leaf for a larger debate to destroy one of the key tenants of the New Deal,” Manley said. “It was a calculated decision that it was time to stand up and fight.”

Now Reid’s digging in against what he sees as an attack on the safety net once more, this time on behalf of a president’s signature initiative. From the looks of it, he’s having the time of his life.

–Suzy Khimm contributed to this report.